Three variously superb pieces of music were performed by the National Symphony Orchestra at its Pension Fund concert last night in the Kennedy Center Concert Hall, but there may have been even more satisfaction in a fourth composition that was not listed on the program.

After about five minutes of standing ovation, hugs, handshakes and repeated bows for guest pianist Rudolf Serkin, the members of the NSO, standing like the audience, picked up their instruments and launched into an impromptu rendition of "Happy Birthday to You." Some orchestra members (not the wind players, of course) sang along as they played, and they were joined by the audience as the music moved toward its climactic "Happy birthday, dear Rudolf." Conductor Mstislav Rostropovich, seeing how well the orchestra did without his guiding baton, flung himself at the keyboard where Serkin had just played Mozart's Piano Concerto No. 19 in F and joined in, making the birthday song a miniature piano concerto.

When he is not busy with his cello or baton, Rostropovich can be a formidable pianist--but, frankly, he was the second-best pianist on the stage last night. Serkin was, as always, meticulous, expressive, beautifully attuned to Mozart's style, engaging readily in dialogue with the orchestra and occasionally, subtly, giving it a small lesson in phrasing and accents. The performance was most effective when the piano and woodwinds were discoursing on the music--partly, no doubt because Serkin's son-in-law, principal oboist Rudolph Vrbsky, was involved in the process. He also played magnificently in the demanding solo oboe part of Rossini's "Silken Ladder" Overture, which opened the program.

It is hardly necessary to say that Shostakovich's Fifth Symphony, which brought the concert to a roof-raising conclusion, was magnificently done. The prowess of Rostropovich and his orchestra in this music has become a fact of life in Washington as prominent as the Washington Monument. Last night's reading richly earned the second standing ovation of the evening. Special mention should be given to the brass and percussion sections, which work very hard in this music and worked splendidly last night. But if one is looking for progress, the heroes of the evening--perhaps of the whole season--were the string section, particularly the violins, which have long been a problem area and are sounding less and less problematic with each passing week. One person largely responsible for the change got his moment in the spotlight when concertmaster William Steck played (very beautifully) the bittersweet violin solo in the scherzo of the Shostakovich.

After the party in the Concert Hall, part of the audience went to a dinner party upstairs in the Roof Terrace Restaurant, where Serkin was presented with a surprise present, inscribed "in honor of his 80th birthday, with love and respect" from the orchestra and Rostropovich: a drawing of himself at the piano and Rostropovich conducting by Washington artist Vint Lawrence. Presenting the gift to "my dearest Rudinka," Rostropovich reflected a moment on the art he shares with Serkin: "God gave to us a marvelous gift, something that makes life longer and better. God saw that Rudolf Serkin gives beauty to many millions of people, and God said to him, 'You give people beauty, and I give you more than 90 years.' "

Serkin, who seemed genuinely surprised by the birthday present, said it was "more than I deserve" and confided privately that he hopes to spend his actual birthday, March 28, quietly with his family, at home in Vermont.