The National Symphony has not played better for any conductor in recent years than Erich Leinsdorf. So it was no surprise that his two weeks this month with the orchestra opened Thursday night in a blaze of glory.

Leinsdorf's conducting here last year of Bruckner's noble Eighth Symphony had been so fine that it made sense to do more Bruckner, in this case the less familiar but somewhat similar Sixth.

Not very many guest conductors can arrive on the scene and with routine rehearsal produce Bruckner playing on this level; this epic music is so taxing, especially on the brass, that results are apt to be rough. But then most conductors do not have Leinsdorf's ear, his precise manner and his exhaustive knowledge of the mechanics of a piece.

Nothing was approximate in Thursday night's playing, as it had been, for instance, when a lesser conductor took on the Seventh Symphony with the NSO earlier this season.

From the very opening one was struck by Leinsdorf's steadiness -- both in the firmness of his tempos and in his control of the overall shape of this lengthy and massive work. Early climaxes were held in check. The enormous range of orchestral textures were notable for the specificity with which they were played (those soft high strings in the second movement were gorgeous).

The second movement was very slow, but Leinsdorf has the control to bring it off that way, and the orchestra was right with him. The final measures, especially, had a haunting repose.

Throughout the symphony, the conductor gave great attention to balances, especially between the strings and the Wagnerian brass choir. That's one of the usual trouble spots, because the brass tends to blare away, producing a harsh sound and covering everyone else. Not once did that happen Thursday night, not even at the end of the last movement, when Leinsdorf finally let the brass really go. Even then the sound was warm and controlled.

Until last year's performances of the Eighth it would not have occurred to one that the National Symphony could be an outstanding Bruckner orchestra, but with one of music's consummate old pros in charge, that's exactly what it is. Leinsdorf is not scheduled to return next season. But he must come back the next year, with more Bruckner; the orchestra clearly loves it, and so does the audience.

Earlier in the program there were two dark-hued works by Bach, played on a chamber scale.

In the Sixth "Brandenburg" Concerto only seven persons played, counting the conductor at the harpsichord. Maybe it wasn't a textbook example of Baroque performance practice, but there were two viole da gamba -- a rare sight at a National Symphony concert. The performance was refined and lightly inflected. Before that came the Sinfonia to the Cantata "Gleichwie der Regen und Schnee," ("Even as the Rain and Snow Fall From Heaven"), BWV 1051 -- also on a chamber scale.