"Welcome back, Slava," the National Symphony Orchestra said last night in the most emphatic way it knows.

The percussion said it with vivid color and incisive rhythms, the woodwinds with smooth phrasing and beautifully blended tone, the strings with a new lustre in their sound and tight ensemble playing, the brass with golden, radiant chords that cut through the Kennedy Center Concert Hall like rays of sunlight. Already, in a still-young season, the orchestra has offered quite a few evenings of special excitement. But last night was more than special; all the stops were pulled out for the sixth season return of Mstislav Rostropovich.

Rostropovich performed in two roles -- as music director and cellist. The evening was a triumph for him in both capacities, but he made it a collective triumph, sharing the warm applause spontaneously with the orchestra, associate conductor Hugh Wolff (who shared the conducting duties) and the two Scandinavian composers whose music had world premieres: Arne Nordheim of Norway and Jon Nordal of Iceland. He stepped back as though taking shelter from the waves of applause and pushed them forward; he motioned to the orchestra to rise and accept its share of credit. There was plenty of applause to go around, and it was richly earned by all parties.

The program was rather daring for such an occasion; none of the music was very familiar to the orchestra or the audience, and it would have been easier to make a strong impression with some of the repertoire that is associated with Rostropovich. To do it with two premieres, along with Grieg's overture, "In Autumn," and with the "Sinfonia Espansiva" (Symphony No. 3) of Carl Nielsen, was a virtuoso tour de force. There was no easy playing to automatic audience preferences, and there was practically no sign that the orchestra had had to master complex new material with limited rehearsal time.

In Nordheim's "Tenebrae" for cello and orchestra, the scenario required the orchestra to give the soloist some rough treatment occasionally -- conflict is an integral part of works in concerto form. And at the proper time, the orchestra did indeed pounce on what the cello had been saying, offer objections and change it to suit its own purposes. It ostracized the instrument, leaving it at the end singing a soft-voiced, high-pitched song of loneliness with only a handful of fellow cellos for company. But that's show biz; Wolff (who conducted this work with superb control) and the orchestra could give their music director no finer tribute than the way they treated the thematic material brought into the discussion by his cello. It was superbly coordinated, dramatic, colorful music presented with strong impact. A large share of the credit for the effectiveness of this music must go naturally to composer Nordheim, who showed an awesome mastery of orchestral sounds and the organization of musical ideas.

The other world premiere was Nordal's "Choralis," which uses Icelandic folk motifs in a very effective contemporary idiom. Like Nordheim, Nordal is a master of pure sound, its textures and contrasts, the tensions it can generate in its interactions and the sheer beauty of unusual blends -- high strings with whispers of percussion, horns cutting through a curtain of strings and woodwinds in astringent harmonies, low winds making a slow, solemn statement. There were a few moments when he could have been taken for an Icelandic Copland, but he quickly established his right to recognition as a totally individual voice.

The most familiar music on the program (though not very familiar) was the Nielsen symphony. It is very much Rostropovich's kind of music -- expansive, as the title indicates, and as spontaneously, unabashedly romantic as any orchestral work composed in this century. Except in the solemn, gentle slow movement, it is music of large gestures -- heated-up tempos, bold statements in the brass and agitated, emphatic work for a large percussion section, which was kept busy all evening.

In a good performance, it can be tremendously exciting, and last night's performance, even with a few, small rough edges, was a very good one: high-voltage from beginning to end, but not roughshod. Some of the finest moments, in the Nielsen as in the other pieces, were the quiet passages. The woodwinds played with particular finesse again and again, and the strings had a special glory in Nielsen's slow movement. That movement also featured excellent wordless singing, with wonderfully atmospheric effect, by soprano Jung Ae Kim and baritone Ben Holt.

If an work showed the pressure that the orchestra must have felt, assimilating and preparing so much unfamiliar material in a short time, it was the opening piece, the Grieg overture. In this, too, the orchestra's sound was impressive but perhaps a shade less nuanced, less precisely together, and the interpretation seemed to emphasize a series of big moments more than thematic continuities. But these were small problems and they are not uncommon in the first work played on an opening night.

In September and October, when the NSO was starting its season under Rafael Fruhbeck de Burgos and Wolff, its music director was in Paris, his other home town. His wife, soprano Galina Vishnevskaya, was singing her farewell to opera in a production of ''Eugene Onegin'' conducted by Rostropovich -- a performance that was spectacularly well received according to reports from Europe. That event may have been more spectacular than what happened here last night. It was opera, which tends to excite the emotions more than the world premieres of Scandinavian music, and it happened in Paris, which is a more excitable city. But in terms of sheer musicianship, accepting and meeting special challenges, it is hard to imagine that it could have been more impressive.