For about 45 minutes last night the National Symphony played like the genuinely great orchestra it is working so hard to become as the 83-year-old Eugene Ormandy led it through a glorious performance of Rachmaninoff's Second Symphony.

True, everything has been mostly uphill for the NSO in recent years, but this listener had not heard it sustain quite the level of sophisticated virtuosity achieved last night.

The orchestra gave Ormandy a degree of inner detail in this incredibly rich orchestral fabric that Ormandy could not always count on even from the Philadelphia Orchestra during his 44 years at that helm. Repeatedly--over those long, soaring, sad Russian melodies--one was hearing subtleties of timbre and supplementary textures that one did not realize were there.

And the violins, as recently as a year and half ago regarded as a weak point of the NSO, were superbly rounded and focused. Their dark sound in the slow movement could have been matched by few other orchestras.

Ensemble throughout was remarkably exact. Note values were precise. Attacks and releases in all the choirs were beautifully coordinated. Time and again phrases were exquisitely molded.

Perhaps what made it possible was a fortuitous intersection of circumstances. Ormandy probably knows this work in greater detail now than anyone has, except his mentor, the composer. And at this point he is considerably more experienced in conducting it than Rachmaninoff was. Meanwhile, under Rostropovich the orchestra has attained a level of assurance in Russian music that it could not match before.

Excellence per se perhaps came as no surprise. But that the aging and ailing Ormandy should achieve such perfection of detail and yet stir such a frenzy of excitement in the second and fourth movements or get such passionate intensity in the slow movement was amazing.

Earlier, Ormandy commanded similar ensemble unity and polish in the Beethoven Egmont Overture and the Mozart 40th Symphony, but the passion wasn't there. One feared that age had taken its toll, until conductor, orchestra and audience became utterly swept up in the Rachmaninoff. There is a repeat tonight and it would be a shame to miss it.