After the intermission of last night's National Symphony concert in the Kennedy Center, Mstislav Rostropovich spoke to the audience. "I have the honor to read to you what the orchestra has said to Isaac Stern."

The conductor then announced that the orchestra had decided unanimously to make Stern an honorary member of the National Symphony in appreciation of his contributions to the world of music. It is an honor that has been given neither often nor lightly. A large plaque bearing the orchestra's appreciation, and signed by every member of the NSO, was presented to Stern while Rostropovich added a word of special tribute to Stern's "special wife, Vera." A lovely, gracious woman who usually remains in the background, Mrs. Stern was present in a center box where she acknowledged the tribute.

Earlier in the day a member of the orchestra said that they had required Stern to audition for his honorary membership. After hearing him play Mozart's G Major Concerto, they agreed that he qualified. On the basis of last night's performance of the radiantly beautiful music, he certainly did, as he has for decades. In a single phrase, that exquisite opening of the slow movement, Stern demonstrated the heights to which his life of making music has carried him.

That is the movement at the beginning of which Mozart wrote in the score, "Here the oboes take up flutes." What follows is one of the heavenly pages in all music and it sounded that way last night from everyone involved. The outer movements went quite as much in the vein, with Stern in superlative form.

His second concerto was the Sibelius in which he is remembered from his earliest recording, made in collaboration with the late Sir Thomas Beecham, one of the historic achievements in all recording history. To its first two movements Stern brought that darkly rich sound for which he is famous, in which he was handsomely accompanied by Rostropovich and the orchestra. For the finale, one of the most tortured and tortuous pages in literature, Stern did what was possible.

The concert, second in the series of five Stern is playing here these weeks, opened with a broadly humorous reading of the Classical Symphony by Prokofiev, in which the humor sounded broader than ever, and the finale, taken at a hair-raising tempo, was a whirlwind success.