Seldom has Mstislav Rostropovich presented such a bounty of string playing as in his programs this week with the National Symphony. Most of it, furthermore, is remarkable in quality -- or at least close to it.

There is cellist Lluis Claret, an almost total unknown here, in Tchalikovsky's Cello Variations on a rococo theme. Then there is the famed Itzhak Perlman in the Menelssohn Violin Concerto. There is, as well, NSO concertmaster Miran Kojian playing solo parts in both the Tchaikovsky "Mozartiana" and in that torrent of musical sensuality, Scriabins's "The Poem of Ectasy."

Claret, for all the bouquet of his evocative last name, is not from France.

He is from that tiny state in the Pyrenees, Andorra, and arrives here by way of something called the Rostropovich Competition, a 1977 contest of which he was a winner.

As a cellist he is a rather different sort of performer from his conductor. The rococo variations are a famous Rostropovich showpiece, and the famous man plays them with immense flourish and enormous dynamic and harmonic nuance.

By contrast, when Claret began them last night, he sounded like a rather matter-of-face player. But -- there were seven variations to go.

By the time they got the statement of the theme by the flute, enchantingly played by Toshiko Kohno, the devilish ascending trills on the celoo were articulated with precision and considerable tone by Claret. Soon came the broad adagio variation in which the cello plays a lament on the main theme. The sound was quite large and the phrasing breathed like a fine contralto's. Clearly, Claret is an important cellist, and is on the verge of becoming an important interpreter.

There need be no such qualifiers, of course, about Perlman any more, particularly when playing the Mendelssohn. The mellow, Lyric character of the work seems made for him. He plays down the scale of the music, keeping in reserve his resources of tone for the special moments -- meanwhile concentration on the flowing line. There are no pyrotechnics for their own sake. Yet the technical command is sensational.

As to cencertmaster Kojian, it was the best playing one has heard from him.

The sound was more full-bodied than usual. In the Tchaikovsky he produced gutsy attacks and carefully pointed phrases.

Also the overall string sound of the orchestra continues to improve. Except when the first violins move into fortissimo, they sound fine indeed. But they have yet to shake a hard edge that often creeps in when they are required to turn on the volume. It robbed "Mozartiana" of some of its elegance and somewhat undercut the lushness of the Scriabin.