UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

FORM 11-K

(Mark One)

Commission File Number: 1-3305
Employer Identification Number: 22-1109110
Plan Number: 004

MERCK & CO., INC. EMPLOYEE STOCK PURCHASE AND SAVINGS PLAN

MERCK & CO., INC.

One Merck Drive
P.O. Box 100
Whitehouse Station, New Jersey 08889-0100

Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Employer Identification Number: 22-1109110
Plan Number: 004
Index

Report of Independent Registered Public Accounting Firm

To the Participants and Administrator of
Merck & Co., Inc. Employee Stock Purchase and Savings Plan

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 Merck & Co., Inc. Employee Stock Purchase and Savings Plan (the “Plan”) at December 31, 2004 and 2003, and the changes in net assets available for benefits for the year ended December 31, 2004 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). Those standards 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.

Our audits were conducted for the purpose of forming an opinion on the basic financial statements taken as a whole. The supplemental Schedule of Assets (Held at Year End) as of December 31, 2004, is presented for the purpose of additional analysis and is not a required part of the basic financial statements but is supplementary information required by the Department of Labor’s Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. The supplemental schedule is the responsibility of the Plan’s management. The supplemental schedule has been subjected to the auditing procedures applied in the audits of the basic financial statements and, in our opinion, is fairly stated in all material respects in relation to the basic financial statements taken as a whole.

Florham Park, New Jersey
June 23, 2005

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Statements of Net Assets Available for Benefits

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

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Statement of Changes in Net Assets Available for Benefits

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

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

1. Description of Plan

The following description of the Merck & Co., Inc. Employee Stock Purchase and Savings Plan (the “Plan”) provides only general information. Participants should refer to the Plan document for a more complete description of the Plan’s provisions.

General

The Plan was designed to provide an easy, economical way for employees to become stockholders of Merck & Co., Inc. (the “Company” or “Merck”) as well as a systematic means of saving and investing for the future. Generally, any regular full-time, part-time, or temporary employee of the Company who is a U.S. resident covered by a collective bargaining agreement providing for participation in this Plan as defined by the Plan document, are eligible to participate in the Plan on or after the first day of the third month following commencement of employment.

The Plan is administered by a management committee appointed by the Company’s Chief Executive Officer or Compensation and Benefits Committee of its Board of Directors.

The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 (“ERISA”).

Master Trust

The assets of the Plan are maintained, for investment purposes only, on a commingled basis with the assets of the Merck & Co., Inc. Employee Savings & Security Plan in the Merck & Co., Inc. Employee Savings & Security Plan and the Merck & Co., Inc. Employee Stock Purchase & Savings Plan Trust (the “Master Trust”). The plans do not own specific Master Trust assets but rather maintain individual beneficial interests in such assets. The portion of fund assets allocable to each plan is based upon the participants’ account balance within each plan. Investment income for each fund is allocated to each plan based on the relationship of each plan’s beneficial interest in the fund to the total beneficial interest of all plans in the fund.

Contributions

Depending on the terms of the applicable collective bargaining agreements, employees earning less than $90,000 are allowed to contribute a maximum of 18% or 25% of base pay. Employees earning $90,000 or more are limited to maximum contributions of 15% of base pay. However, pre-tax contributions shall not exceed the 2004 IRS limit of $13,000. In addition, the Company will match 60% or 65% of employee contributions up to 6% of base pay per pay period as negotiated with each bargaining group. Company matching contributions are invested according to the following age parameters:

Under age 50 — 50% of Company matching contributions are invested in the Merck Common Stock Fund (non-participant directed) and 50% are invested in the funds to which the participant is currently contributing (participant directed). Since January 2005, all participants have had the option to invest all Company matching contributions in any of the available fund options (participant directed), except the Medco Health Common Stock Fund.

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

Age 50 and above — Participants have the option to invest all Company matching contributions in any of the available fund options (participant directed), except the Medco Health Common Stock Fund. Also, the Economic Growth and Tax Relief Reconciliation Act of 2001 permits catch-up contributions that are designed to provide individuals age 50 and above with an additional pre-tax retirement savings opportunity. As such, eligible participants in the Plan could contribute an additional $3,000 for 2004.

Participants direct the investment of their contributions into any mutual fund investment option available under the Plan as well as Merck Common Stock (participant directed). During 2004, the Plan offered 17 mutual funds, the Medco Health Common Stock Fund and the Merck Common Stock Fund.

Participant Accounts

Each participant’s account is credited with the participant’s contribution, the Company’s matching contribution and allocation of Plan earnings. The allocation is based on participants’ account balances, as defined in the Plan document.

Vesting

Participants are immediately vested in their contributions, all Company matching contributions, plus actual earnings thereon.

Participant Loans

Participants may borrow from their account balances with interest charged at the prime rate plus 1%. Loan terms range from one to five years for a short term loan or up to thirty years for the purchase of a primary residence. The minimum loan is $500 and the maximum loan is the lesser of $50,000 less the highest outstanding loan balance(s) during the one year period prior to the new loan application date, or 50% of the participant’s account balance less any current outstanding loan balance and defaulted loan amounts.

Payment of Benefits

In-service (which include hardship withdrawals) and termination distributions are made throughout the year in accordance with applicable Plan provisions.

2. Summary of Accounting Policies

Basis of Accounting

The accompanying financial statements are prepared on the accrual basis of accounting.

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 revenues and expenses during the reporting period. Management believes that these estimates are adequate. Actual results could differ from those estimates.

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Notes to Financial Statements

Total investment income of the Master Trust for the year ended December 31, 2004 is as follows:

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Merck & Co., Inc.
Employee Stock Purchase and Savings Plan

Schedule H

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SIGNATURE

The Plan. Pursuant to the requirements of the Securities Exchange Act of 1934, the undersigned plan administrator has duly caused this annual report to be signed on behalf of the Merck & Co., Inc. Employee Stock Purchase and Savings Plan by the undersigned hereunto duly authorized.

June 27, 2005

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EXHIBIT INDEX

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