In a concert marked by beauty and excitement, the National Symphony Orchestra last night introduced a magnificent new artist to Washington.

She is Cecile Licad, 20, and blessed with a gift for making music gloriously on the piano. Last January, Licad won the Leventritt Competition, the first person to do so in 10 years. Previous winners Eugene Istomin and Van Cliburn were in the audience last night to hear their dazzling successor.

In the Saint-Sae ns G Minor Concerto, Licad was a perpetual wonder. Every sound she made was beautiful, every note and phrase the result of intellect warmed by emotion. The great solo opening was magisterially announced with the deliberation of a mature artist. With a singing tone as exquisite as her appearance, she combines the poetry of a Myra Hess with the easy power of a Gina Bachauer.

The end of the first movement, with its interlocking octaves taken in stunning precision, was electrifying. At the end of the scherzo, the audience could not resist applauding, and at the conclusion of the concerto the house rose to its feet with thunderous shouts, the musicians in the orchestra joining in.

Rostropovich and the orchestra, in virtuoso form, matched their young colleague with brilliant fire, while the strings made their entrance, after the first movement's cadenza, a moment of magic.

The evening opened with a tribute to composer-critic Virgil Thomson, who will be 85 in three weeks. Rostropovich led his music for the film, "The Plow That Broke the Plains" in a lovely, lyrical mood, letting the banjo and saxophone supply just the right flavor.

To cap off one of the NSO's finest evenings, the orchestra was joined by the women of the Oratorio Society and ideal soloists Linda Mabbs and Janice Felty in most of the music Mendelssohn wrote for "A Midsummer Night's Dream." Everything came out beautifully, with Rostropovich showing the kind of personal involvement and affection that Sir Thomas Beecham used to shower on the radiant score.