**Exhibit 31.2****CERTIFICATION**     I, Judy C. Lewent, certify that:     1. I have reviewed this quarterly report on Form 10-Q of Merck & Co., Inc.;     2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit tostate a material fact necessary to make the statements made, in light of the circumstances under which suchstatements were made, not misleading with respect to the period covered by this report;     3. Based on my knowledge, the financial statements, and other financial information included in thisreport, fairly present in all material respects the financial condition, results of operations and cash flows ofthe registrant as of, and for, the periods presented in this report;     4. The registrant’s other certifying officer(s) and I are responsible for establishing and maintainingdisclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internalcontrol over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrantand have:     a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures tobe designed under our supervision, to ensure that material information relating to the registrant, includingits consolidated subsidiaries, is made known to us by others within those entities, particularly during theperiod in which this report is being prepared;     b) Designed such internal control over financial reporting, or caused such internal control over financialreporting to be designed under our supervision, to provide reasonable assurance regarding the reliability offinancial reporting and the preparation of financial statements for external purposes in accordance withgenerally accepted accounting principles;     c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in thisreport our conclusions about the effectiveness of the disclosure controls and procedures, as of the end ofthe period covered by this report based on such evaluation; and     d) Disclosed in this report any change in the registrant’s internal control over financial reporting thatoccurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in thecase of an annual report) that has materially affected, or is reasonably likely to materially affect, theregistrant’s internal control over financial reporting; and     5. The registrant’s other certifying officer(s) and I have disclosed, based on our most recent evaluationof internal control over financial reporting, to the registrant’s auditors and the audit committee of theregistrant’s board of directors (or persons performing the equivalent functions):     a) All significant deficiencies and material weaknesses in the design or operation of internal control overfinancial reporting which are reasonably likely to adversely affect the registrant’s ability to record,process, summarize and report financial information; and     b) Any fraud, whether or not material, that involves management or other employees who have a significantrole in the registrant’s internal control over financial reporting.     Date: August 7, 2006       By: /s/ Judy C. Lewent         JUDY C. LEWENT    Executive Vice President & Chief Financial Officer