UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                               WASHINGTON DC 20549


                                    FORM 11-K




                   (X) ANNUAL REPORT PURSUANT TO SECTION 15(d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

                   FOR THE FISCAL YEAR ENDED DECEMBER 31, 2003

                                       OR

                 ( ) TRANSITION REPORT PURSUANT TO SECTION 15(d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934
                        For the transition period from to
                             ---------------- -----

                          COMMISSION FILE NUMBER 1-14756



          A.   Full title of the plan and the address of the plan,  if different
               from that of the issuer named below:



                           AMEREN CORPORATION EMPLOYEE
                      LONG-TERM SAVINGS PLAN - IUOE NO. 148
         (formerly known as the Central Illinois Public Service Company
                Employee Long - Term Savings Plan - IUOE No. 148



          B.   Name of issuer of  securities  held  pursuant to the plan and the
               address of its principal executive office:

                               Ameren Corporation
                              1901 Chouteau Avenue
                            St. Louis, Missouri 63103













                Ameren Corporation
                Employee Long-Term
                Savings Plan - IUOE No. 148
                Financial Statements
                December 31, 2003 and 2002










Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Index
December 31, 2003 and 2002
----------------------------------------------------------------------------------------------------------------------
                                                                                                        

                                                                                                            Page(s)

Report of Independent Registered Public Accounting Firm...........................................................1

Financial Statements

Statements of Net Assets Available for Benefits...................................................................2

Statements of Changes in Net Assets Available for Benefits........................................................3

Notes to Financial Statements...................................................................................4-8




Note:     Schedules  required by 29 CFR 2520.103-10 of the Department of Labor's
          Rules and Regulations for Reporting and Disclosure  under the Employee
          Retirement  Income  Security Act of 1974 ("ERISA"),  as amended,  have
          been omitted because they are not applicable.






             Report of Independent Registered Public Accounting Firm



To the Participants and Administrator of the
Ameren Corporation Employee Long-Term
Savings Plan - IUOE No. 148



In our opinion, the accompanying statements of net assets available for benefits
and the  related  statements  of changes in net assets  available  for  benefits
present fairly, in all material respects,  the net assets available for benefits
of the Ameren  Corporation  Employee  Long-Term Savings Plan - IUOE No. 148 (the
"Plan") at December 31, 2003 and 2002,  and the changes in net assets  available
for benefits for the years then ended in conformity with  accounting  principles
generally accepted in the United States of America.  These financial  statements
are the  responsibility  of the  Plan's  management.  Our  responsibility  is to
express  an  opinion  on these  financial  statements  based on our  audits.  We
conducted our audits of these statements in accordance with the standards of the
Public Company Accounting Oversight Board (United States), which require that we
plan and  perform the audit to obtain  reasonable  assurance  about  whether the
financial  statements  are free of  material  misstatement.  An  audit  includes
examining,  on a test basis,  evidence supporting the amounts and disclosures in
the  financial   statements,   assessing  the  accounting  principles  used  and
significant  estimates made by management,  and evaluating the overall financial
statement  presentation.  We believe that our audits provide a reasonable  basis
for our opinion.

As  described  in  Note 3 to the  financial  statements,  the  Plan's  financial
statements   do   not   disclose    certain    information   with   respect   to
nonparticipant-directed  Plan assets held by the Plan's  trustee.  Disclosure of
this  information is required by the Department of Labor's Rules and Regulations
for Reporting and Disclosure under the Employee  Retirement  Income Security Act
of 1974.





St. Louis, Missouri
June 18, 2004


                                        1








Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Statements of Net Assets Available for Benefits
December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------


                                                                                       2003               2002
                                                                                           
Investments in Central Illinois Public Service Company
 Master Long-Term Savings Trust (Note 6)*                                          $ 27,808,210       $ 25,024,721

Cash                                                                                      5,365              4,064

Receivables
    Participant contributions                                                            54,660             95,911
    Employer contributions                                                               13,563             15,063
    Dividends and interest                                                               17,777             24,128
    Due from broker for securities sold                                                  20,410                120
                                                                                ----------------  -----------------

             Total receivables                                                          106,410            135,222
                                                                                ----------------  -----------------

             Net assets available for benefits                                      $27,919,985        $25,164,007
                                                                                ================  =================


* Represents five percent or more of net assets available for benefits.











    The accompanying notes are an integral part of the financial statements.

                                        2







Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Statements of Changes in Net Assets Available for Benefits
Years Ended December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------------
                                                                                     2003               2002
                                                                                           
Investment income (loss)
Plan interest in Central Illinois Public Service Company
 Master Long-Term Savings Trust (Note 6)                                            $ 4,048,610        $(1,562,639)
                                                                                ----------------  -----------------

Contributions
Participant contributions                                                             1,343,618          1,518,204
Employer contributions                                                                  212,395            243,204
                                                                                ----------------  -----------------

                                                                                      1,556,013          1,761,408
                                                                                ----------------  -----------------

Benefits and expenses
Benefits paid to participants                                                         2,537,315          1,052,292
Administrative expenses                                                                   1,820              2,080
                                                                                ----------------  -----------------

                                                                                      2,539,135          1,054,372
                                                                                ----------------  -----------------

Plan transfer out (Note 1)                                                              309,510             57,056
                                                                                ----------------  -----------------

             Net increase (decrease)                                                  2,755,978           (912,659)

Net assets available for benefits
Beginning of the year                                                                25,164,007         26,076,666
                                                                                ----------------  -----------------

End of the year                                                                     $27,919,985        $25,164,007
                                                                                ================  =================









    The accompanying notes are an integral part of the financial statements.

                                        3




Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

1.   Description of the Plan

     General
     The  following is a brief  summary of the various  provisions of the Ameren
     Corporation (the "Company")  Employee Long-Term Savings Plan - IUOE No. 148
     (the "Plan"). The Plan provides for the investment in certain funds by each
     participating employee (the "Participants") who are members of the IUOE No.
     148 collective bargaining unit employed by Ameren Energy Generating Company
     ("Participating  Subsidiary"), an indirectly wholly owned subsidiary of the
     Company.  Participants  should refer to the Plan document for more complete
     information.

     The Company  adopted the Plan on January 1, 1990,  to provide a  systematic
     means by which  certain  eligible  employees  of the  Company  may  adopt a
     regular  savings  program and secure federal income tax benefits  resulting
     from  participation  in the Plan. The Plan is a defined  contribution  plan
     subject to certain provisions of ERISA, as amended,  and regulations of the
     Securities and Exchange Commission.

     The  Company  serves as sponsor  of the Plan,  and,  consequently,  has the
     authority to amend or terminate  the Plan subject to certain  restrictions.
     The Board of Directors of the Company has the authority and  responsibility
     for the  general  administration  of the Plan.  Merrill  Lynch & Co.,  Inc.
     ("Merrill Lynch"), as Trustee, has the authority and responsibility to hold
     and protect the assets of the Plan in accordance  with Plan  provisions and
     the separate Trust Agreement.

     Certain   reclassifications  have  been  made  to  prior  year's  financial
     statements to conform to 2003 reporting.

     Participation
     Each employee of the Participating  Subsidiary  receiving regular salary or
     wages who is part of the IUOE Local No. 148 collective  bargaining unit and
     who has  both  completed  one year of  service  (defined  as a  consecutive
     twelve-month period beginning with his/her employment  commencement date or
     anniversary  thereof during which he/she has completed at least 1,000 hours
     of service)  and has attained the age of 21 is eligible to become an active
     participant.

     Contributions
     Prior to December 30, 2002,  Participants could contribute from one percent
     to 15  percent  of their  base  compensation  to the Plan  through  payroll
     deductions (basic contributions).  Effective December 30, 2002, the Company
     amended the Plan to allow  Participants  to  contribute up to the lesser of
     one to 100 percent of their base  compensation  or $12,000  annually to the
     Plan through payroll deductions (basic contributions).  The Company makes a
     matching contribution equal to $.25 for each $1.00 on the first six percent
     of a Participant's  contribution  (basic match  contribution).  The Company
     will  contribute an additional $.05 for each $1.00 on the first six percent
     of a  Participant's  contribution to be invested in the Ameren Common Stock
     Fund (additional matching contribution). Company matching contributions are
     made based on specific  agreements  between the Company and the  collective
     bargaining unit. A portion of Company matching contributions is invested in
     the Ameren  Common Stock Fund.  All Company  contributions  are made to the
     extent  sufficient  earnings  are  available,  as  described  in  the  Plan
     document.


                                        4




Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     Participants  direct  their basic  contributions  and the  Company's  basic
     matching  contributions by electing that such  contributions be placed in a
     single  investment  fund or allocated in  increments  of one percent to any
     combination of available  investment funds. Such fund allocation  elections
     may be changed  daily.  Earnings  derived from the assets of any investment
     fund are  reinvested  in the fund to which they  relate.  Participants  may
     elect daily to reallocate  all or in one percent  increments,  the value of
     their  accounts  between funds.  Pending  investment of the assets into any
     investment  fund,  the  Trustee may  temporarily  make  certain  short-term
     investments.

     Participant Loans
     The Plan  permits  Participants  to borrow from their  accounts  within the
     Plan. Such borrowings may be made subject to the following: (1) the minimum
     amount of the loan is $500,  (2) the  amount of the loan may not exceed the
     lesser  of  $50,000  or  fifty   percent  of  the  vested   amount  in  the
     Participant's  account,  (3) the loan will bear a fixed  interest  rate and
     repayments  will  be made  through  mutual  agreement  subject  to  certain
     statutory  repayment time limits,  and (4) such other rules and regulations
     as may be  adopted by the  Company.  At  December  31,  2003 and 2002,  the
     interest rates on Participant  loans ranged from 4.0 percent to 9.5 percent
     and 4.25 percent to 9.5 percent, respectively.

     Vesting
     The amounts in Participants' accounts, including Company contributions, are
     fully vested at all times.

     Payment of Benefits
     Upon  termination  of  employment  for any reason,  a  Participant  will be
     entitled  to receive  the  balance in the  Participant's  account  less the
     unpaid  amount  of  any  outstanding  loan,   including  accrued  interest.
     Generally,  distributions  will be made in a lump sum; however,  in certain
     circumstances a Participant may also elect to receive his/her  distribution
     in installments.  Certain distributions may be deferred until a Participant
     reaches age 70 1/2,  dies, or requests an earlier  distribution,  whichever
     occurs first.

     Plan Transfer Out
     Plan  transfers out represent  Participants'  account  balances  which were
     transferred  from the Plan  during  the year  into the  Ameren  Corporation
     Savings Investment Plan.

     Plan Termination
     The Company intends to continue the Plan indefinitely. However, the Company
     has a right to terminate the Plan at any time subject to the  provisions of
     ERISA.  Upon  termination,  the Trustee will distribute assets remaining in
     the Plan with the  exception  that no  distributions  shall be made until a
     Participant attains age 59 1/2, except in certain specified situations.

2.   Summary of Significant Accounting Policies

     Basis of Accounting
     The  accompanying  financial  statements  of the Plan are  prepared  on the
     accrual basis of accounting,  except that benefit  payments to participants
     are recorded upon distribution.

                                        5




Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     Use of Estimates
     The  preparation  of financial  statements  in conformity  with  accounting
     principles  generally  accepted  in the United  States of America  requires
     management  to make  estimates  and  assumptions  that affect the  reported
     amounts of assets and liabilities  and disclosure of contingent  assets and
     liabilities  at the  date of the  financial  statements  and  the  reported
     amounts  of  changes  in net  assets  available  for  benefits  during  the
     reporting period. Actual results could differ from those estimates.

     Investments
     All investments  held by the Master Trust are presented at fair value as of
     December 31, 2003 and 2002.  The fair value of the Ameren Common Stock Fund
     was determined  using  year-end  published  market  prices.  Investments in
     equity securities and bonds are valued at published net asset market values
     including accrued income on the last business day of each year. Investments
     in the Money Market Fund and Merrill Lynch  Retirement  Preservation  Trust
     are  valued  at  cost  plus  accrued  income,  which  approximates  market.
     Participant loans are valued at cost, which approximates fair market value.

     Investment  securities are exposed to various risks, such as interest rate,
     market,  and  credit.  Due to the  level of risk  associated  with  certain
     investment  securities and the level of  uncertainty  related to changes in
     the value of investment securities, it is at least reasonably possible that
     changes  in risks in the near term  could  materially  affect  the  amounts
     reported in the Statement of Net Assets Available for Benefits.

     Income
     Interest  income is  recorded  on the  accrual  basis.  Dividend  income is
     recorded on the ex-dividend date.

     Gains and losses on security  transactions  are recorded on the trade date.
     Net unrealized  appreciation or  depreciation  for the year is reflected in
     net  appreciation  (depreciation)  in  fair  value  of  investments  in the
     Statement of Changes in Net Assets Available for Benefits.

     Administrative Expenses
     In general, expenses to administer the Plan, including fees and expenses of
     the Trustee,  are paid by the Company,  except as provided for in the Plan.
     All transaction fees of an investment fund are paid from the assets of that
     investment fund.

3.   Nonparticipant-Directed Investments

     Employer contributions relating to the nonparticipant-directed  investments
     which  included  Company stock were $35,647 and $41,195 for the years ended
     December 31, 2003 and 2002,  respectively.  Other information about the net
     assets    and   the    changes   in   net    assets    relating    to   the
     nonparticipant-directed  investments  is not disclosed as this  information
     was not able to be provided by the Plan's trustee.

4.   Transactions with Parties-in-Interest

     At December  31, 2003,  the Master Trust held Company  common stock for the
     Plan  with  a  cost  and  market  value  of  $8,546,690   and   $9,411,947,
     respectively.  During  2003,  the  Plan  purchased  shares  at  a  cost  of
     $2,139,023 and sold shares valued at $2,617,812.

                                        6




Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Notes to Financial Statements
December 31, 2003 and 2002
--------------------------------------------------------------------------------

     At December  31, 2002,  the Master Trust held Company  common stock for the
     Plan  with  a  cost  and  market  value  of  $7,815,839   and   $9,180,269,
     respectively.  During  2002,  the  Plan  purchased  shares  at  a  cost  of
     $2,525,804 and sold shares valued at $2,679,288.

     At December 31, 2003, the Master Trust held investments in various accounts
     for the Plan that are  related to Merrill  Lynch,  the Plan's  Trustee.  At
     December  31,  2003,  these  investments  had a cost  and  market  value of
     $8,480,017  and  $9,788,490,  respectively.  At December  31,  2002,  these
     investments  had a cost and  market  value of  $7,508,284  and  $7,237,974,
     respectively.

     These  transactions  are  allowable  party-in-interest  transactions  under
     Section 408(b)(8) of the ERISA regulations.

5.   Federal Income Tax Status

     The Company  received a favorable  determination  letter from the  Internal
     Revenue  Service dated May 29, 2002,  concerning the  qualification  of the
     Plan under federal income tax  regulations.  In addition,  the Company also
     received a favorable determination letter from the Internal Revenue Service
     dated December 8, 1986,  concerning  qualification  of the Central Illinois
     Public Service Company Master Long-Term  Savings Trust under federal income
     tax regulations.  Management  believes that the Plan is currently  designed
     and is being  operated in  compliance  with  requirements  of the  Internal
     Revenue Code and that the Plan is tax exempt as of the financial  statement
     date.


6.   Participation in Master Trust

     The Central Illinois Public Service Company Master Long-Term  Savings Trust
     (the "Master Trust") was established April 1, 1985, to serve as the funding
     vehicle for the Plan and for the other separate Employee  Long-Term Savings
     Plan,  which is for the members of the IBEW No. 702  collective  bargaining
     unit employed by Central  Illinois Public Service  Company,  a wholly owned
     subsidiary of the Company, and the Participating Subsidiary.  This separate
     plan is not  included  in this  report and is shown  separately  in its own
     report.  At  December  31,  2003 and 2002,  the Plan's  interest in the net
     assets of the Master  Trust was  approximately  41 percent  and 43 percent,
     respectively.   Investment   income   (loss),   gains   and   losses,   and
     administrative  expenses  relating to the Master Trust are allocated to the
     Participant's  account balance based on the percentage of the Participant's
     account balance to the total of all Participants'  account balances in each
     investment fund as of each valuation date.


                                        7





Ameren Corporation
Employee Long-Term Savings Plan - IUOE No. 148
Notes to Financial Statements
December 31, 2003 and 2002
------------------------------------------------------------------------------------------------------------------


     The following  table presents the fair value of investments  for the Master
     Trust at December 31, 2003 and 2002:


                                                                                  2003              2002
                                                                                      
     Investments at fair value
     Ameren Common Stock Fund                                                 $ 21,121,177      $ 19,223,384
     Standard & Poor's 500 Equity Index Trust                                   14,761,489        11,556,668
     Growth Equity Fund                                                          9,010,167         6,713,396
     Merrill Lynch Retirement Preservation Trust                                 5,570,523         3,575,048
     Money Market Fund                                                           5,157,889         5,490,256
     AIM Value Fund                                                              4,365,812         3,472,562
     Bond Index Fund                                                             3,238,826         3,204,261
     Participant Loan Fund                                                       2,361,913         2,485,781
     Merrill Lynch Global Allocation Fund - Class A                              1,732,344         1,060,384
     Merrill Lynch Capital Fund - Class A                                        1,242,312           782,614
                                                                           ----------------  ----------------

             Total Master Trust investments                                   $ 68,562,452      $ 57,564,354
                                                                           ================  ================





     Investment  income (loss) for the Master Trust for the years ended December
     31, 2003 and 2002, is as follows:




                                                                                  2003              2002

                                                                                      
     Interest and dividends                                                   $  1,628,387      $  1,648,516
     Net appreciation (depreciation) in fair value of investments                9,026,400        (6,783,629)
                                                                           ----------------  ----------------

                                                                              $ 10,654,787      $ (5,135,113)
                                                                           ================  ================



                                        8





                                   SIGNATURES

          The Plan.  Pursuant to the requirements of the Securities Exchange Act

of 1934,  the trustees (or other  persons who  administer  the employee  benefit

plan)  have duly  caused  this  annual  report to be signed on its behalf by the

undersigned hereunto duly authorized.


                                         AMEREN CORPORATION EMPLOYEE
                                         LONG-TERM SAVINGS
                                         PLAN - IUOE NO. 148


                                         AMEREN SERVICES COMPANY
                                             (Administrator)




                                         By   /s/ Donna K. Martin
                                           -------------------------------------
                                                  Donna K. Martin
                                                  Vice President

June 28, 2004




                                  EXHIBIT INDEX



Exhibit No.                       Description
-----------        -----------------------------------------

   23              Consent of Independent Auditors