A startling thing happened toward the end of Tchaikovsky's Pathetique Symphony that closed yesterday afternoon's National Symphony concert. At the stroke of the gong that introduced the coda, Mstislav Rostropovich stopped conducting. While the trombone quartet played the ensuing passage, the conductor stood motionless. Only as the strings were to begin the solemn receding music that closed the symphony did he raise his baton and resume his direction.

It was a dramatic closing to a magnificent reading. The outer movements began and ended as Tchaikovsky directed they should, very slowly. But the allegro section of the opening movement went with mounting fire and the endlessly fascinating 5/4 "waltz" was full of enchantments. The martial scherzo rode in headlong brilliance, flawlessly building to its final explosion.

The National Symphony this week is playing all of the music it will soon present in some of the most famous concert halls in the world: the Berlin Philharmonic, home of Herbert von Karajan and the Berlin Philharmonic; the Amsterdam Concertgebouw; the Salle Pleyel in Paris; above all, Vienna's glorious Grosse Musikvereinsaal, home of the Vienna Philharmonic and a hall once frequented by Brahms, Bruckner, Mahler and Richard Strauss.

Yesterday's program repeated Barber's "School for Scandal" overture, which sounded both more assured and more spontaneous than on the previous night.

The Eighth Symphony of Beethoven was, for the most part, a great pleasure. Rostropovich paced it beautifully, keeping to the fore its classic mold in contrast to the symphonies with which Beethoven preceded and followed it.

Along with some beautiful playing, particularly in the delicacy of its quieter pages, there were some unexpected rough spots. These were perhaps due to the burden of playing three different programs in four concerts in four days. The orchestra leaves for Europe on Monday, and will return Feb. 28.