Mahler's Sixth Symphony, which the National Symphony played last night under Mstislav Rostropovich, is the composer's most unrelievedly pessimistic work. As Mahler's protege', Bruno Walter, observed, more than any of the other nine symphonies, it shouts "No." All of them are concerned above all with the spiritual and the metaphysical, but only here -- especially in the shattering finality of the concluding pages -- does even the slightest glimmer of hope elude this most philosophical of music's giants.

The Sixth is enormous -- about an hour and a half long, with the largest and most diverse scoring of any nonchoral Mahler work (including Alpine cowbells, a huge Olympics-style hammer to deliver the fatal musical coups de gra ce at the end, and a rute, a birch rod with which to hit the bass drum).

The Sixth is also overwhelming in its cumulative intensity. One reason is that the structure is so tight, its four movements more strictly classical than those of any other symphony by Mahler. Another reason is that, in a good performance, the pulse never easily wavers -- not even in the superficially directionless Alpine interludes that seem to come unconsciously out of nowhere in the first and third movements.

The classical side of the the Sixth was served well in last night's performance -- the first of this season's concluding week of concerts. Rostropovich had the work's structure easily in hand and tended to play it straight. It was an interpretation marked by vigor and steadiness, with tempos that were moderate in the turbulent sections. The interpretation tended to go right at this pace without bending as the music proceeded into other parts of the most rapturous lyricism (as in Rostropovich's rather stiff phrasing of the opening movement's second theme, a gorgeous love melody that Mahler wrote in tribute to his wife, the legendary Alma).

What lingers most hauntingly in the mind, though, about the Sixth is its poetry -- with its sudden and almost impressionist shifts of mood and atmosphere. It is in this dimension that the symphony, for all its structural rationalism, develops its real character. In this sense, it is a matchless portrait of a mind hurtling through unconscious stages toward an increasingly consuming doom -- and all of this psychological expression written seven or eight years before the historic session between Mahler and Freud.

To really do ultimate justice to the range of this material -- tonally and instrumentally razor-sharp at one moment and ineffably delicate the next -- was beyond last night's enterprise.

Ideally, the Sixth demands an instant grace, purity and suavity that Rostropovich and his players did not attain. Examples of the problem: the delicacies of Mahler's intricate percussion scoring were sometimes drowned out; the complexities of the symphony's polyphonic detail were sometimes obscured, and their articulation sometimes blurred even when heard.

In this work, these reservations are not nit-picking. The Mahler Sixth calls for consummate virtuosity from a symphony orchestra. And that kind of playing is hard to achieve under the National Symphony's tight rehearsal conditions.

It was clear, though, that Rostropovich and the orchestra had given the symphony their full time and energy. That must have been the reason for the last-minute cancellation of the "Good Friday Spell" from Wagner's "Parsifal," which was to start the evening. It would have been yet another considerable challenge; and, in any case, the Mahler Sixth is plenty of music for one concert