MCKESSON CORPORATION

                                                                    EXHIBIT 32.1

                            CERTIFICATION PURSUANT TO
                             18 U.S.C SECTION 1350,
      AS ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002


In connection with the quarterly report of McKesson Corporation (the "Company")
on Form 10-Q for the quarter ended September 30, 2003 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), the
undersigned, in the capacities and on the dates indicated below, each hereby
certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906
of the Sarbanes-Oxley Act of 2002, that to the best of their knowledge:

(1)   The Report fully complies with the requirements of section 13(a) or 15(d)
      of the Securities Exchange Act of 1934; and

(2)   The information contained in the Report fairly presents, in all material
      respects, the financial condition and results of operations of the
      Company.

/s/ John H. Hammergren
-------------------------------
JOHN H. HAMMERGREN
Chief Executive Officer
October 30, 2003

/s/ William R. Graber
-------------------------------
WILLIAM R. GRABER
Chief Financial Officer
October 30, 2003

      This certification accompanies the Report pursuant to Section 906 of the
Sarbanes-Oxley Act of 2002, and shall not, except to the extent required by the
Sarbanes-Oxley- Act of 2002, be deemed filed by the Company for the purposes of
Section 18 of the Securities Exchange Act of 1934, as amended.

      A signed original of this written statement required by Section 906 has
been provided to McKesson Corporation and will be retained by the Company and
furnished to the Securities and Exchange Commission or its staff upon request.