Last night, Mstislav Rostropovich and the National Symphony Orchestra took two neglected stepchildren of the modern Russian repertoire--Rachmaninoff's Fourth Piano Concerto and Shostakovich's Seventh Symphony--and gave them the performance of a lifetime. In the Rachmaninoff, Eugene Istomin's solo piano performance was a revelation.

If a fresh, totally dedicated interpretation could confer greatness on a piece of music, both works would have achieved greatness last night in the Kennedy Center Concert Hall. In fact, they emerged as exciting, interesting music that should be heard more often in such performances--but not as the composers' best work. Each is a sort of transition to things that the composer said later with more clarity, economy and focus--Shostakovich in his 13th and 14th symphonies, Rachmaninoff in his Rhapsody on a Theme of Paganini.

The Shostakovich Seventh in this week's performances might well be dedicated to the passengers on Korean Air Lines' ill-fated Flight 007. Originally composed during and about the Nazi siege of Leningrad, it is, among other things, an elegy for the victims of fascist brutality. This works equally well whether the fascism has a German or a Russian accent. In 1956, Shostakovich's Seventh could have been the "Budapest" Symphony rather than the "Leningrad"; in 1968, it might have been called "Prague," and today the symbol that best matches its emotional impact is its original number, slightly modified: "007." At its most powerful moments, it is a statement worthy of that episode--and there are not many others in music.

Rostropovich and the orchestra generated the most power in the passage where the confrontation of brutality and innocence is most graphically portrayed--a long, slow march in Shostakovich's first movement that builds gradually to a shattering climax and stays in the listener's memory long after the sounds have faded away. Interrupting an idyllic pastoral passage, it begins as the merest hint of a whisper in the strings, over an obsessive snare-drum rhythm, and it is repeated, with more and more instruments joining in, the pulse becoming more insistent, the volume rising to a deafening level, until it comes to grief in a violent encounter with a Russian folk song.

It may be interesting to note that the melodic nucleus of the march is a melody from "The Merry Widow": "I'm going to Maxim's." As Shostakovich began to compose the music in Leningrad, it may have seemed that his 3-year-old son, Maxim, embodied the values and hopes that were so brutally threatened. As it turned out, another brand of fascism disrupted Maxim Shostakovich's life. He is now a defector from the Soviet Union.

There are some grounds for thinking that Shostakovich may have had Stalin in mind as well as Hitler when he wrote this symphony, though Maxim Shostakovich is one of the people who doubt this interpretation. In "Testimony," his controversial memoirs as told to Solkomon Volkov, Shostakovich says the symphony was "planned before the war" and that he "was thinking of other enemies of humanity when I composed the theme . . . I feel eternal pain for those who were killed by Hitler, but I feel no less pain for those killed on Stalin's orders." In any case, the Nazi invasion was certainly the occasion for the music but its emotional impact applies to other forms of brutality.

Rostropovich's interpretation was both thoughtful and powerful throughout both pieces--a bit understated at the beginning of the Rachmaninoff while he was seeking an ideal balance with the soloist, but quite powerful and assured thereafter. Istomin emphasized the "American" elements in the music more than any other performer I have heard, without disguising its essentially Russian soul, and it was most effective.

Both compositions could have profited from some tightening before they were first made public. But last night, they had a kind of power seldom seen before.