EXHIBIT 24

POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS THAT each of the undersigned directors and officers of McKesson Corporation, a Delaware corporation (the “Company”), does hereby constitute and appoint Jeffrey C. Campbell, Laureen E. Seeger and Willie C. Bogan his or her true and lawful attorney and agent, each with full power and authority (acting alone and without the other) to execute in the name and on behalf of the undersigned as such director and/or officer, under the Securities Exchange Act of 1934, as amended, an Annual Report on Form 10-K for the fiscal year ended March 31, 2010, and thereafter to execute and file any and all amendments to such Form . Each of the undersigned hereby grants unto such attorneys and agents, and each of them, full power of substitution or resubstitution and revocation in the premises and hereby ratifies and confirms all that such attorneys and agents may do or cause to be done by virtue of these presents.

/s/ Andy D. Bryant	/s/ Marie L. Knowles
Andy D. Bryant, Director	Marie L. Knowles, Director
/s/ Wayne A. Budd	/s/ David M. Lawrence
Wayne A. Budd, Director	David M. Lawrence, Director
/s/ Jeffrey C. Campbell	/s/ Edward A. Mueller
Jeffrey C. Campbell, Executive Vice President	Edward A. Mueller, Director
and Chief Financial Officer
(Principal Financial Officer)
/s/ John H. Hammergren	/s/ Nigel A. Rees
John H. Hammergren, Chairman of the Board,	Nigel A. Rees, Vice President and Controller
President and Chief Executive Officer	(Principal Accounting Officer)
(Principal Executive Officer)
/s/ Alton F. Irby	/s/ Jane E. Shaw
Alton F. Irby III, Director	Jane E. Shaw, Director
/s/ M. Christine Jacobs
M. Christine Jacobs, Director

Dated: April 21, 2010