SEPRACOR

                                 PROMISSORY NOTE

$150,000                                                                1/22/98

         FOR VALUE RECEIVED, the undersigned PAUL D. RUBIN, M.D. with an address
of 37 Greystone Lane, Sudbury, MA 01776 (hereinafter referred to as the
"Borrower"), promises to pay to the order of Sepracor Inc. (together with any
subsequent holders of this Note, the "Lender"), at its office at 111 Locke
Drive, Marlborough, Massachusetts 01752, or at such other place as the Lender
may from time to time designate in writing, the principal sum of:

                       ONE HUNDRED FIFTY THOUSAND DOLLARS

         If not sooner paid, all outstanding principal shall be paid to the
Lender on the earlier of (a) date of termination from employment; (b) upon
exercise of 12,000 stock options at the end of the SEC blackout period; (c) June
30, 1998.

         THIS NOTE IS INTEREST-FREE and may be prepaid, in whole or from time to
time in part, at anytime, without premium or penalty. ALL TAXES RELATIVE TO THIS
INTEREST-FREE LOAN ARE THE RESPONSIBILITY OF THE BORROWER.

         All payments hereunder shall be payable in lawful money of the United
States which shall be legal tender for public and private debts at the time of
payment.

         It is expressly agreed that the occurrence of any one or more of the
following shall constitute an "Event of Default" hereunder: (a) failure to pay
upon termination of employment (b) failure to pay note on expiration. If any
such Event of Default hereunder shall occur, the Lender may, at its option,
declare to be immediately due and payable the then outstanding principal balance
under this Note, and all other amounts payable to the Lender hereunder, shall
become and be due and payable immediately. The failure of the Lender to exercise
said option to accelerate shall not constitute a waiver of the right to exercise
the same at any other time.

         The Borrower will pay on demand all costs and expenses, including
reasonable attorneys' fees, incurred or paid by the Lender in enforcing or
collecting any of the obligations of the Borrower hereunder. The Borrower agrees
that all such costs and expenses and all other expenditures by the Lender on
account hereof, other than advances of principal, which are not reimbursed by
the Borrower immediately upon demand, all amounts due under this Note after
maturity, and any amounts due hereunder if an Event of Default shall occur
hereunder, shall bear interest at a fluctuating per annum rate equal to the sum
of the Prime Rate from time to time in effect plus five percent, but in no event
more than the maximum rate of interest then permitted by law (the "Default
Rate"), until such expenditures are repaid or this Note
<PAGE>   2
and such amounts as are due are paid to the Lender.

         THIS NOTE IS SECURED BY ALL OF THE BORROWERS STOCK OPTIONS IN SEPRACOR
INC. UPON TERMINATION OF BLACKOUT PERIOD, BORROWER IS REQUIRED TO SELL 12,000
STOCK OPTIONS AS REPAYMENT AGAINST LOAN. NO SALE, TRANSFER, OR ASSIGNMENT OF
SAID STOCK OPTIONS CAN BE MADE WITHOUT THE EXPRESS WRITTEN CONSENT OF THE SR.
VICE PRESIDENT, FINANCE & ADMINISTRATION, OR THE CEO OF SEPRACOR WHILE THIS LOAN
IS OUTSTANDING.

         All notices required or permitted to be given hereunder shall be in
writing and shall be effective when mailed, postage prepaid, by registered or
certified mail, addressed in the case of the Borrower and the Lender to them at
the address set forth above, or to such other address as either the Borrower or
the Lender may from time to time specify by like notice.

         All of the provisions of this Note shall be binding upon and inure to
the benefit of the Borrower and the Lender and their respective successors and
assigns. If there is more than one undersigned Borrower, the obligations of each
Borrower shall be joint and several. This Note shall be governed by and
construed in accordance with the laws of The Commonwealth of Massachusetts.

         The Borrower hereby consents to any extension of time of payment
hereof, release of all or any part of the security for the payment hereof, or
release of any party liable for this obligation, and waives presentment for
payment, demand, protest and notice of dishonor. Any such extension or release
may be made without discharging its liability.

IN WITNESS WHEREOF, the Borrower has executed and delivered this Note, on the
day and year first above written.



 /s/Robert F. Scumaci                            /s/Paul D. Rubin
 --------------------                           -------------------------------
Robert F. Scumaci                               Paul D. Rubin, M.D.
Sr. Vice President                              Sr. Vice President, Pharma. Dev.
Finance & Administration

Source: OneCLE Business Contracts.