Exhibit
10.45      March 20, 2003  VIA HAND DELIVERY  Shawn M. OConnor Pharsight Corporation  Dear Shawn:  As we have discussed with
you, Pharsight Corporation (the Company) in conjunction with Execustaff,
Inc., is pleased to offer you a promotion to the position of Chief Executive
Officer under the terms set forth in this letter.  Your promotion and the terms set forth in this letter agreement
(Agreement) shall be effective as of February 13, 2003.  Promotion and New Position  You will be promoted from
your current position of Chief Operating Officer to the position of Chief
Executive Officer.  In your new
position, you will report to the Companys Board of Directors ( the Board).  Your areas of responsibility will include
general management of and responsibility for the entire Company, and any other
tasks requested by the Board.  Your
office will continue to be located at the Companys headquarters in Mountain
View, California.  The Company continues
to retain the discretion to change your position, duties, reporting
relationship and work location, as it deems necessary.  Board Membership  In
connection with your promotion, you were elected to the Board effective
February 13, 2003.  Base Salary and Bonus Potential  As a result of your
promotion, your annual base salary will be increased to two hundred eighty
thousand dollars ($280,000), subject to standard payroll deductions and
required withholdings, and paid on the Companys normal payroll schedule.  Pursuant to the terms and
conditions of the Companys Management Incentive Bonus Program for its
Executive Officers, you will be eligible to receive an annual performance bonus
for Fiscal Year 2003 (FY2003) targeted at fifty percent (50%) of your
previous and increased base salaries, each on a pro rata basis, subject to
standard payroll deductions and required withholdings.  The Companys Compensation Committee will
determine in its sole discretion whether you have earned an annual bonus for
FY2003, and the amount of any earned annual bonus, provided that, if you are actively employed as an employee
in good standing as of the date that the FY2003 bonuses are paid, you are
guaranteed to receive a FY2003 bonus of at least $30,000, subject to standard
payroll deductions and required withholdings.  The Company may modify
your compensation from time to time as it deems necessary.   1 
   Stock Options  Your current stock
options are not affected by your promotion or this Agreement, and your stock
option agreements will remain in full force and effect in accordance with their
terms.  Subject to Board approval, the
Company may grant you additional stock options at the next regularly scheduled
meeting in April 2003.  Your September 2002 stock
option grant of 250,000 shares will fully vest upon a Change in Control as
defined below.  Employee Benefits  Your eligibility for
Company-sponsored employee benefits is not affected by your promotion or this
Agreement.  Proprietary Information and
Inventions Agreement; Company Policies and Procedures  Your Proprietary Information and Inventions Agreement with the Company
dated September 9, 2002 (the Proprietary Information Agreement) isnot affected by your promotion or this
Agreement, and the Proprietary Information Agreement will remain in full force
and effect in accordance with its terms. 
You will continue to be required to abide by the Proprietary Information
Agreement as a condition of your employment.  In addition, you will
continue to be required to abide by Pharsights policies and procedures, as may
be in effect from time to time.  At-Will Employment Relationship  Your employment continues
to be terminable at-will, and either you or the Company may terminate your
employment relationship at any time, with or without Cause (defined below) or
advance notice.  Severance Benefits In the event that your employment is involuntarily terminated by the
Company without Cause, as your sole severance benefits, for twelve (12) months
following the termination date, the Company will continue to pay your base
salary in effect on the termination date (the Severance Payments) and will
pay the costs to continue your health care coverage under COBRA at the same
level of coverage as in effect as of the termination date if you timely elect continued
health care coverage (collectively, the Severance Benefits).  As a condition of your receipt of the
Severance Benefits, you must first enter into a separation agreement with the
Company that includes your general release of all known and unknown claims, in
a form provided by the Company, and you must resign from your membership on the
Board with such resignation to be effective on or before your employment
termination date.  The Severance
Payments will be paid on the Companys normal payroll schedule and will be
subject to standard deductions and withholdings. For the purposes
of this letter, Cause for your termination shall mean:  (a) your conviction of any felony or of
any crime involving dishonesty; (b) your participation in any fraud or act
of dishonesty against the Company; (c) the material breach of your duties
to the Company, including persistent unsatisfactory performance of job duties;
(d) your intentional damage to, or willful misappropriation of, any
property of the Company; (e) your material breach of any written agreement with
the Company (including this  2 
   Agreement or your
Proprietary Information Agreement); or (f) conduct, that in the good faith and
reasonable determination of the Board demonstrates gross unfitness to serve. In addition, if,
after a Change in Control (defined below), you resign from your employment with
the Company and such resignation qualifies as a Resignation for Good Reason
(defined below), you shall be entitled to receive the Severance Benefits, provided that you must first enter into a
separation agreement with the Company that includes your general release of all
known and unknown claims, in a form provided by the Company, and you must
resign from your membership on the Board with such resignation to be effective
on or before your employment termination date. For the purposes
of this letter, the occurrence of
either of the following events shall constitute a Change in Control:  (a) the sale or lease of all or substantially
all of the assets of the Company; or (b) an acquisition of the Company by
another corporation or entity by consolidation, merger or other reorganization
in each case in which the holders of the Companys outstanding voting stock
immediately prior to such transaction own, immediately after such transaction,
securities representing less than fifty percent (50%) of the voting power of
the corporation or other entity purchasing such assets or surviving such
transaction. For purposes of
this letter, a Resignation for Good Reason shall mean a resignation by you
due to any of the following events which occur after and as a direct result of
a Change in Control: (1) a material reduction in compensation, unless such
a reduction is applied, by resolution of the Board of Directors, to all members
of the Companys officers; (2) a material adverse change in your title due to a
demotion; or (3) a material adverse reduction in your role and
responsibilities. You will not be
eligible for any severance benefits in the event of a termination with Cause or
any resignation that does not qualify as a Resignation for Good Reason. If the
relationship between Execustaff and the Company is terminated for any
reason, you will agree that the Company will become solely responsible as
your employer for all payroll, workers compensation and benefits, including
severance and vacation pay, and you will agree to seek the same only
from the Company. Miscellaneous  This Agreement sets forth
and forms the complete and exclusive statement of your employment agreement
with the Company concerning your promotion and employment terms, and this
Agreement supersedes any other agreements or promises made to you by anyone,
whether oral or written, concerning the subject matters set forth in this
letter including, but not limited to, your original offer letter with the
Company dated September 4, 2002.  This
letter agreement cannot be changed except in a writing signed by you and a duly
authorized member of the Board, and such writing must be approved by the
Compensation Committee to be effective.    3 
   We are very pleased to
offer you this promotion.  Please sign
and date this letter, and return it to me by March __, 2003, if you wish to
accept this promotion under the terms described above.  Sincerely,Pharsight
  Corporation/s/  Arthur ReidelArthur ReidelChairman of the Board of DirectorsACCEPTED:/s/ 
  Shawn M. OConnor
  
  March 31, 2003Shawn M. OConnor
  
  Date       4