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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<PAGE>

                                                                   Exhibit 10.14

                                 PROMISSORY NOTE

$22,750.00                                                 Palo Alto, California
                                                                   June 15, 1999

      FOR VALUE RECEIVED, the undersigned hereby unconditionally promises to pay
to the order of PHARSIGHT CORPORATION, a California corporation (the "Company"),
at 800 West El Camino Real, Suite 200, Mountain View, CA 94040, or at such other
place as the holder hereof may designate in writing, in lawful money of the
United States of America and in immediately available funds, the principal sum
of twenty-two thousand seven hundred fifty Dollars ($22,750.00) together with
interest accrued from the date hereof on the unpaid principal at the rate of 6%
per annum, or the maximum rate permissible by law (which under the laws of the
State of California shall be deemed to be the laws relating to permissible rates
of interest on commercial loans), whichever is less, as follows:

Principal Repayment. The outstanding principal amount hereunder shall be due and
payable in full on May 1, 2003 (the "Principal Repayment Date"); and

Interest Payments. Interest shall be payable in arrears on the Principal
Repayment Date and shall be calculated on the basis of a 360-day year for the
actual number of days elapsed;

provided, however, that in the event that the undersigned's employment by or
association with the Company or its Affiliate is terminated for any reason prior
to payment in full of this Note, this Note shall be accelerated and all
remaining unpaid principal and interest shall become due and payable immediately
after such termination.

      If the undersigned fails to pay any of the principal and accrued interest
when due, the Company, at its sole option, shall have the right to accelerate
this Note, in which event the entire principal balance and all accrued interest
shall become immediately due and payable, and immediately collectible by the
Company pursuant to applicable law.

      This Note may be prepaid at any time without penalty. All money paid
toward the satisfaction of this Note shall be applied first to the payment of
interest as required hereunder and then to the retirement of the principal.

      The full amount of this Note is secured by a pledge of shares of Common
Stock of the Company, and is subject to all of the terms and provisions of the
Early Exercise Stock Purchase Agreement and Stock Pledge Agreement of even date
herewith between the undersigned and the Company.

      The undersigned hereby represents and agrees that the amounts due under
this Note are not consumer debt, and are not incurred primarily for personal,
family or household purposes, but are for business and commercial purposes only.
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      The undersigned hereby waives presentment, protest and notice of protest,
demand for payment, notice of dishonor and all other notices or demands in
connection with the delivery, acceptance, performance, default or endorsement of
this Note.

      The holder hereof shall be entitled to recover, and the undersigned agrees
to pay when incurred, all costs and expenses of collection of this Note,
including without limitation, reasonable attorneys' fees.

      This Note shall be governed by, and construed, enforced and interpreted in
accordance with, the laws of the State of California, excluding conflict of laws
principles that would cause the application of laws of any other jurisdiction.

                                    Signed            /s/ Robin Kehoe
                                            ------------------------------

                                                        Robin Kehoe<PAGE>
                                                                   Exhibit 10.15

                             SECURED PROMISSORY NOTE

$75,000                                                        January 25, 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 Seventy-Five Thousand dollars ($75,000), together with interest
thereon at the rate of 5.93% 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) December 17, 2002 or (ii) 90 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 300,000 shares (the
"Shares") of the Common Stock of Holder, pursuant to the terms of that certain
Employee Restricted Stock Purchase Agreement of even date herewith 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, upon five (5) business
days notice of any such event which shall not have been cured during such five
(5) business day period, 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.

                                       1
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         In Witness Whereof, this Note has been duly executed and delivered by
Borrower on the date first above written.

                                                  /s/ Daniel Weiner
                                              -----------------------------
                                                      Daniel Weiner

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