1
                                                                     Exhibit 5.1



                              PALMER & DODGE LLP  
                   ONE BEACON STREET, BOSTON, MA 02108-3190


Telephone: (617) 573-0100                             Facsimile: (617) 227-4420

                              December 18, 1996

Cambridge NeuroScience, Inc.
One Kendall Square, Building 700
Cambridge, Massachusetts 02139

Ladies and Gentlemen:

        We are rendering this opinion in connection with the Registration
Statement on Form S-3 (the "Registration Statement") filed by Cambridge
NeuroScience, Inc. (the "Company") with the Securities and Exchange Commission
under the Securities Act of 1933, as amended, on or about the date hereof. The
Registration Statement relates to up to 2,300,000 shares of the Company's
Common Stock, $0.001 par value (the "Shares"). We understand that the Shares
are to be offered and sold in the manner described in the Registration 
Statement.

        We have acted as your counsel in connection with the preparation of the
Registration Statement. We are familiar with the resolutions adopted by the
Board of Directors by written consent on December 17, 1996 in connection with
the authorization, issuance and sale of the Shares (the "Resolutions"). We have
examined such other documents as we consider necessary to render this opinion.

        Based upon the foregoing, we are of the opinion that the Shares have
been duly authorized and, when issued and delivered by the Company against
payment therefor at the price to be determined pursuant to the Resolutions,
will be validly issued, fully paid and non-assessable.

        We hereby consent to the filing of this opinion as a part of the
Registration Statement and to the reference to our firm under the caption
"Legal Matters" in the Prospectus filed as part thereof.

                                        Very truly yours,




                                        /s/ Palmer & Dodge LLP
                                        -----------------------
                                        Palmer & Dodge LLP