In a brief discussion during the intermission of last night's National Symphony concert, guest conductor Erich Leinsdorf observed of the group, "This is definitely an orchestra on the way up. The question is simply how far they are going to go up. There is no question about it."

High praise, indeed, from the man who for years conducted the Boston Symphony and is one of the most esteemed of all conductors.

But it is not to be assumed that even he was prepared for the orchestral tour de force that the audience heard after intermission in the performance of Bruckner's 9th, and last, symphony. It is a work of gargantuan proportions, with an intensely philosophic sense of finality about it -- which is to say, few performances ever do it justice. Last night's did.

Just programming the work as a guest with an orchestra that has supposedly not yet reached the highest level was a gutsy move on Leinsdorf's part. But, as it turned out it was quite a coup because the sonorous, brilliant results suggested that the National Symhony is not as far from the top as most persons assume.

Just take the brass playing, which is at the heart of this symphony. The bite in the repeated triplets of the scherzo was virtually matchless. In my own experience, only the Berlin Philharmonic has exceeded it -- likewise, the string pizzacati in the same movement.

The admiration of the National Symphony players for Leinsdorf is well known. But never had he tested them as last night. And it was the first time they had been put to such a test since this year's enlargement of the strings and improvement of the winds and brasses. If ever there was a doubt about Mstislav Rostropovich's hard personnel decision, last night should remove it.

Rostropovich's great contribution has been to reform the sound of the orchestra. But one got the feeling that the Leinsdorf week is a test they've been waiting for. It is their finest hour. Along with the Bruckner there is a brisk, majestic performance of the Mozart "Jupiter" Symphony. There are repeats for the next three evenings.