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                                                                     EXHIBIT 2.1

                                FIRST AMENDMENT
                                        
                                        

     FIRST AMENDMENT, dated as of December 16, 1998 (the "First Amendment"), by
and among SUMMIT DESIGN, INC., a Delaware corporation ("Parent"), HOOD
ACQUISITION CORP., a Delaware corporation and wholly-owned subsidiary of Parent
("Merger Sub"), and ORCAD, INC., a Delaware corporation (the "Company" and
together with Parent and Merger Sub, the "Parties"),


     WHEREAS, the Parties have entered into an Agreement and Plan of
Reorganization dated as of September 20, 1998 (the "Reorganization Agreement";
the terms defined in the Reorganization Agreement being used in this First
Amendment as so defined unless otherwise defined herein);

     WHEREAS, the Parties desire to extend the expiration date of the
Reorganization Agreement without otherwise amending it or waiving any rights or
conditions contained in or based on it;

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants
and agreements herein contained, and intending to be legally bound hereby, the
Parties hereby agree as follows:

     1.  First Amendment to Reorganization Agreement
         -------------------------------------------
 
         (a) Section 7.1 (b) of the Reorganization Agreement is hereby amended
to change the reference to "January 31, 1999" to "February 28, 1999".

         (b)  Except as expressly modified and amended above, the Reorganization
Agreement shall remain in full force and effect and is hereby ratified and
confirmed.

     2.  Execution in Counterparts.  This First Amendment may be executed in one
         -------------------------
or more counterparts, each of which when executed shall be deemed to be an
original but all of which taken together will constitute one and the same
agreement.

     3.  Governing Law.  This First Amendment shall be governed by and construed
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in accordance with the laws of the State of Delaware, regardless of the laws
that might otherwise govern under applicable principles of law thereof.
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     IN WITNESS WHEREOF, each of the Parties has executed this First Amendment
to the Reorganization Agreement as of the date first above written.


                                       SUMMIT DESIGN, INC.


                                       By:  /s/ LARRY J. GERHARD
                                          -----------------------------       
                                            Name:  Larry J. Gerhard
                                            Title: President and CEO


                                       HOOD ACQUISITION CORP.


                                       By:  /s/ LARRY J. GERHARD
                                          ----------------------------
                                            Name:  Larry J. Gerhard
                                            Title: President and CEO


                                       ORCAD, INC.


                                       By:  /s/ MICHAEL F. BOSWORTH
                                          ----------------------------
                                            Name:  Michael F. Bosworth
                                            Title: President and CEO