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                                                                   Exhibit 10.13

                             Secured Promissory Note

$10,000.00                                                          June 2, 1998

      For Value Received, the undersigned ("Borrower") hereby promises to pay to
the order of Pharsight Corporation, a California corporation ("Holder"), the
principal sum of twelve thousand dollars ($10,000.00), together with interest
thereon at the rate of 5.77% per annum, compounded annually, on the unpaid
balance of such principal amount from the date hereof. The principal hereof, and
all accrued and unpaid interest thereon, shall be due and payable on the earlier
of (i) July 25, 2001, or (ii) 30 days after the termination of Borrower's
employment by Holder. The principal hereof, and accrued interest thereon, may be
prepaid at any time, in whole or in part, without premium or penalty.

      Payments of principal and interest on this Secured Promissory Note (this
"Note") shall be made in legal tender of the United States of America and shall
be made at the office of Holder at 299 California Avenue, Suite 300, Palo Alto,
CA 94306 or at such other place as Holder shall have designated in writing to
Borrower. If the date set for any payment on this Promissory Note is a Saturday,
Sunday or legal holiday, then such payment shall be due on the next succeeding
business day.

      As of the date hereof, Borrower has purchased 40,000 shares (the "Shares")
of the Common Stock of Holder, pursuant to the terms of that certain Restricted
Stock Purchase Agreement dated as of June 2, 1998 by and between Borrower and
Holder. This Note shall be secured by the Shares as provided in that certain
Stock Pledge Agreement (the "Pledge Agreement") of even date herewith by and
between Holder and Borrower.

      In the event Borrower shall (i) fail to make complete payment of principal
or accrued interest when due under this Note or (ii) commit a breach of, or
default under, the Pledge Agreement, Holder may accelerate this Note and declare
the entire unpaid principal amount of this Note and all accrued and unpaid
interest thereon to be immediately due and payable and, thereupon, the unpaid
principal amount and all such accrued and unpaid interest shall become and be
immediately due and payable, without notice of default, presentment or demand
for payment, protest or notice of nonpayment or dishonor or other notices or
demands of any kind (all of which are hereby expressly waived by Borrower). The
failure of Holder to accelerate this Note shall not constitute a waiver of any
of Holder's rights under this Note as long as Borrower's default under this Note
or breach of or default under the Pledge Agreement continues.

      The provisions of this Note shall be governed by, and construed in
accordance with, the laws of the State of California without regard to the
conflicts of law rules thereof. In the event that Holder is required to take any
action to collect or otherwise enforce payment of this Note, Borrower agrees to
pay such attorneys' fees and court costs as Holder may incur as a result
thereof, whether or not suit is commenced.

      In Witness Whereof, this Note has been duly executed and delivered by
Borrower on the date first above written.

     /s/ Robin Kehoe
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      Robin Kehoe