1 EXHIBIT 10.1 AMENDMENT TO PROMISSORY NOTE ---------------------------- THIS AMENDMENT dated at of October 29, 1999, (the "Amendment") is made by and between Dr. Douglas Reed (the "Borrower") and GelTex Pharmaceuticals, Inc., a Delaware corporation...

   1
                                  EXHIBIT 10.1

                          AMENDMENT TO PROMISSORY NOTE
                          ----------------------------

     THIS AMENDMENT dated at of October 29, 1999, (the "Amendment") is made by
and between Dr. Xxxxxxx Xxxx (the "Borrower") and GelTex Pharmaceuticals, Inc.,
a Delaware corporation (the "Lender") and amends the Promissory Note in the
amount of $100,000 executed in Waltham, Massachusetts and dated December 1,
1998.


                                 R E C I T A L S

     WHEREAS, Dr. Xxxxxxx Xxxx and GelTex Pharmaceuticals, Inc. are parties to
that certain Promissory Note dated December 1, 1998;

     WHEREAS, Dr. Xxxxxxx Xxxx and GelTex Pharmaceuticals, Inc. desire to amend
the Promissory Note to extend the Maturity Date of said note;

     NOW, THEREFORE, the parties mutually agree as follows:

     The Maturity Date of this Promissory Note shall be the earlier of (i)
fifteen days following the closing date of the Borrower's sale of the property
located at 0000 X. Xxxxxxxxx Xxxxx, Xxxx Xxxx Xxxx, Xxxx or (ii) March 31, 2000.

     Executed as a sealed instrument as of the day and year first above written.

BORROWER:

 /s/ Xxxxxxx Xxxx
----------------------------
Xxxxxxx Xxxx

LENDER:

GELTEX PHARMACEUTICALS, INC.

By:

 /s/ Xxxx X. Xxxxxxx, Xx.
---------------------------
Xxxx X. Xxxxxxx, Xx., Vice President
Finance and Administration