Excitement was the keynote of last night's National Symphony Orchestra concert, from the opening cadenza for two harps (beautifully played by Dotian Carter and Alyce Rideout) to the final bars of the encore: "Hail to the Redskins," played with the audience clapping along in unison. It was a high-voltage evening with barely a letup, but its most exciting part was not the well-selected program or the highly kinetic conducting of Mstislav Rostropovich. It was the playing of cellist Frans Helmerson, a young artist of enormous potential making his first appearance with the National Symphony.

It must require considerable courage to be the soloist in the Dvorak Cello Concerto with Rostropovich, the world's leading cellist for this music, on the podium. But simply getting such an assignment requires great ability. Helmerson has both in good measure and much more.

Above all, he has a tone of breathtaking beauty, capable of many subtle variations in nuance, and he shapes it into gorgeous, long, spun-out singing lines that are ideal for Dvorak's enchanting melodies. His technique is precise and seems effortless; his stage presence is pleasant and a bit self-effacing--a condition that may change as audiences around the world become more aware of his talent. He does not yet have the kind of dynamic range that Rostropovich commands as a cellist, and that may explain why the orchestra outbalanced the soloist in a few passages--a problem that should disappear in repeat performances.

Except for "Hail to the Redskins," all the music played last night was Czech; besides the Dvorak, there was Smetana's folk-flavored "Vysehrad" from "Ma Vlast" and Janacek's inventive Sinfonietta, which was moved to the last position on the program, where the Dvorak had originally been scheduled. The performance--brilliant and highly individual but a shade short of eccentric--fully justified this climactic placement. Rostropovich made the music his own, exulting in idioms that are quite close to his own Russian heritage.

In his emphasis on bright details of its colors, he sometimes made the Sinfonietta sound more loose-knit than it really is. But if he treated it a little bit like Russian ballet music of the 1920s, this is an approach well-calculated to bring out the music's vivid, exotic flavors and probably one that Janacek (who was given to pan-Slavic mysticism) would have approved. The orchestra played magnificently.