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                                   EXHIBIT 4.2

                                 PROMISSORY NOTE

$3,000,000.00                                              Boston, Massachusetts
                                                                 October 4, 1999


     FOR VALUE RECEIVED, the undersigned GelTex Pharmaceuticals, Inc., a
Delaware corporation (the "Borrower") hereby promises to pay to the order of
FLEET NATIONAL BANK (the "Bank") the principal amount of Three Million and
00/100 ($3,000,000.00) Dollars or such portion thereof as may be advanced by the
Bank pursuant to ss.1.2 of that certain letter agreement of even date herewith
between the Bank and the Borrower (the "Letter Agreement") and remains
outstanding from time to time hereunder ("Principal"), with interest, at the
rate hereinafter set forth, on the daily balance of all unpaid Principal, from
the date hereof until payment in full of all Principal and interest hereunder.

     Interest on all unpaid Principal shall be due and payable monthly in
arrears, on the first day of each month, commencing on the first such date after
the advance of any Principal and continuing on the first day of each month
thereafter and on the date of payment of this note in full, at a fluctuating
rate per annum (computed on the basis of a year of three hundred sixty (360)
days for the actual number of days elapsed) which shall at all times (except as
described in the next sentence) be equal to the Prime Rate, as in effect from
time to time (but in no event in excess of the maximum rate permitted by then
applicable law), with a change in the aforesaid rate of interest to become
effective on the same day on which any change in the Prime Rate is effective;
provided, however, that if all or any portion of outstanding Principal is
represented by a LIBOR Revolving Loan (as defined in the Letter Agreement) for
any Interest Period (as defined in the Letter Agreement), then interest for such
Interest Period on such LIBOR Revolving Loan shall be payable at the applicable
LIBOR Interest Rate (determined as provided in the Letter Agreement) on the
Interest Payment Date (as defined in the Letter Agreement) applicable to such
Interest Period. After the occurrence and during the continuance of any Event of
Default (as defined in the Letter Agreement), interest will accrue and be
payable under this note at a fluctuating rate per annum which at all times shall
be equal to the sum of (i) four (4%) percent per annum plus (ii) the per annum
rate otherwise payable under this note (but in no event excessive of the maximum
rate permitted by then applicable law). As used herein, "Prime Rate" means that
variable rate of interest per annum designated by the Bank from time to time as
its prime rate, it being understood that such rate is merely a reference rate
and does not necessarily represent the lowest or best rate being charged to any
customer. If the entire amount of any required Principal and/or interest is not
paid within ten (10) days after the same is due, the Borrower shall pay to the
Bank a late fee equal to five percent (5%) of the required payment.

     All outstanding Principal and all interest accrued thereon shall be due and
payable in full on the first to occur of: (i) an acceleration under ss.5.2 of
the Letter Agreement or (ii) September 30, 2002. The Borrower may at any time
and from time to time prepay all or any portion of said


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Principal, without premium or penalty, but, as to LIBOR Revolving Loans, only at
the times and in the manner, and with the yield maintenance fee (if any),
provided for in the Letter Agreement. Under certain circumstances set forth in
the Letter Agreement, prepayments of Principal may be required.

     Payments of both Principal and interest shall be made, in lawful money of
the United States in immediately available funds, at the office of the Bank
located at One Federal Street, Boston, Massachusetts 02110, or at such other
address as the Bank may from time to time designate.

     The undersigned Borrower irrevocably authorizes the Bank to make or cause
to be made, on a schedule attached to this note or on the books of the Bank, at
or following the time of making any Revolving Loan (as defined in the Letter
Agreement) and of receiving any payment of Principal, an appropriate notation
reflecting such transaction and the then aggregate unpaid balance of Principal.
Failure of the Bank to make any such notation shall not, however, affect any
obligation of the Borrower hereunder or under the Letter Agreement. The unpaid
Principal amount of this note, as recorded by the Bank from time to time on such
schedule or on such books, shall constitute presumptive evidence of the
aggregate unpaid principal amount of the Revolving Loans.

     The Borrower hereby (a) waives notice of and consents to any and all
advances, settlements, compromises, favors and indulgences (including, without
limitation, any extension or postponement of the time for payment), any and all
receipts, substitutions, additions, exchanges and releases of collateral, and
any and all additions, substitutions and releases of any person primarily or
secondarily liable, (b) waives presentment, demand, notice, protest and all
other demands and notices generally in connection with the delivery, acceptance,
performance, default or enforcement of or under this note, and (c) agrees to pay
all costs and expenses, including, without limitation, reasonable attorneys'
fees, incurred or paid by the Bank in enforcing this note and any collateral or
security therefor, all whether or not litigation is commenced.

     This note is the Revolving Note referred to in the Letter Agreement. This
note is secured by, and is entitled to the benefits of, the Security Agreement
(as defined in the Letter Agreement). This note is subject to prepayment (with a
yield maintenance fee consequent thereon in certain cases, as more fully
described in the Letter Agreement) as set forth in the Letter Agreement. The
maturity of this note may be accelerated upon the occurrence of an Event of
Default, as provided in the Letter Agreement.

     THE BORROWER HEREBY KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVES THE
RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY CLAIM BASED ON THIS NOTE OR ARISING
OUT OF, UNDER OR IN CONNECTION WITH THIS NOTE OR ANY RELATED DOCUMENTS OR OUT OF
ANY COURSE OF CONDUCT, COURSE OF DEALING, STATEMENTS (WHETHER ORAL OR WRITTEN)
OR ACTIONS OF ANY PERSON. THIS WAIVER CONSTITUTES A MATERIAL


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INDUCEMENT FOR THE BANK TO ACCEPT THIS NOTE AND TO MAKE LOANS AS CONTEMPLATED IN
THE LETTER AGREEMENT.

     Executed, as an instrument under seal, as of the day and year first above
written.

CORPORATE SEAL                                 GELTEX PHARMACEUTICALS, INC.

ATTEST:

  /s/ Elizabeth A. Grammer                     By: /s/ Paul J. Mellett
----------------------------                      --------------------
Secretary                                         Name:  Paul J. Mellett
                                                  Title: Chief Financial Officer


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