Exhibit 99(a)





                                  CERTIFICATION


         Pursuant to 18 U.S.C. Section 1350, the undersigned officer of Merck &
Co., Inc. (the "Company"), hereby certifies that the Company's Quarterly Report
on Form 10-Q for the quarter ended June 30, 2002 (the "Report") fully complies
with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act
of 1934 and that the information contained in the Report fairly presents, in all
material respects, the financial condition and results of operations of the
Company.



Dated:  August 13, 2002                   /s/ Raymond V. Gilmartin
                                          -----------------------------------
                                 Name:    Raymond V. Gilmartin
                                 Title:   Chairman, President and Chief
                                          Executive Officer