**Exhibit 31.1****CERTIFICATION**     I, Richard T. Clark, 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 to state a material factnecessary to make the statements made, in light of the circumstances under which such statements were made, not misleading withrespect to the period covered by this report;     3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present inall material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periodspresented in this report;     4. The registrant’s other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls andprocedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined inExchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have:     a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under oursupervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is madeknown to us by others within those entities, particularly during the period in which this report is being prepared;     b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to bedesigned under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and thepreparation of financial statements for external purposes in accordance with generally accepted accounting principles;     c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report ourconclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this reportbased on such evaluation; and     d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during theregistrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that hasmaterially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and     5. The registrant’s other certifying officer(s) and I have disclosed, based on our most recent evaluation of internal controlover financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or personsperforming the equivalent functions):     a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reportingwhich are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financialinformation; and     b) Any fraud, whether or not material, that involves management or other employees who have a significant role in theregistrant’s internal control over financial reporting.     Date: August 7, 2006       By: /s/ Richard T. Clark         RICHARD T. CLARK    Chief Executive Officer and President