The National Symphony gave the world premiere last night of a big, ambitious new symphony by the Finnish composer Aulis Sallinen. This was the 32-minute Symphony No. 5, "Washington Mosaics," Op. 57. It was the second premiere within four years of a splashy orchestral by this composer that music director Mstislav Rostropovich has conducted here.

On first blush -- or rather first burst, given its sonic dimensions -- the work is more intriguing than convincing. There are genuinely moving moments, such as the warm, gorgeously harmonized four-note theme that provides a sort of epilogue to the largely cacophonous first of the five movements. Lovely as it is, though, the passage does not in any way seem to flow naturally out of the rhythmic, harmonic or melodic material before it. There is no rule, of course, that it must. Otherwise the glorious final moments of the first movement of Tchaikovsky's "Pathe'tique" would be suspect. But unlike the "Pathe'tique," it is unclear what Sallinen's passage is meant to resolve emotionally. It is merely beautiful -- which is more than I can say for the often brilliant passages that lead to it. They are mostly timbral virtuosity, with a percussion battery like you've seldom seen, or heard, before.

The overall upshot of this five-movement work (actually, two aggressive outer movements and three languorous inner "intermezzos") is more one of fragments than of normal cumulative development. Sallinen seems to underscore this in Richard Freed's notes: "The most essential thing people carry with them when they leave the concert hall -- perhaps without being aware of it -- is the structure of the composition."

And he goes on this way, seeming to suggest that this is more a narrative symphony than an organic one. Yet what is the narrative? He discounts any programmatic implications of the subtitle, except to say that "it sounds beautiful to me . . . Furthermore, Washington is not just any city in the world: it is the headquarters of one of the two world powers of today, with all its might -- and responsibility." So be it, but does that really shed light on why the work goes the way it does?

Nonetheless, the symphony is compactly organized, and quite specific in its parts. There was so much music that recalled the works of the composer's most famous countryman, Sibelius (brass fanfares, lush strings, bleak solo winds and those motor chords in the massed winds), that one sometimes wondered if it were not a valedictory to the Finnish master.

One thing was sure. Rostropovich had the NSO rehearsed down to the last dotted "i." It sounded truly fine.

Perhaps an inevitable result was the less precise playing in Sibelius' "En Saga" and in the concluding Beethoven Second Symphony. The Sibelius was adequate, but this early work needs all the help it can get.

The Beethoven was weightier and had less wit than it might. But it was a perfectly reasonable and responsible way with a work so wonderful that it lends itself to many varying approaches. And Rostropovich's slow movement was lovely by any standard -- a considerable achievement alone.