Mstislav Rostropovich took on a new role last night when, before conducting Stravinsky's "Petrouchka," he told his National Symphony audience the story of the ballet and illustrated some of its chief episodes with the help of the orchestra.

"Excuse me, I want speak," he began. This is story of village festival at Shrove time, time when Russians eat a lot, time before sturgeon in Caspian Sea seeking political asylum. All this is present in the music -- real Russian carnival with dancing, an organ grinder. Listen to his organ with the squeaky reeds."

As Rostropovich continued in his dazzling display of Russianized English, the audience laughed and applauded. "At peak of festival is drum, demanding attention of audience. There is puppet theater; comes magician, charlatan, but in spite of his magic, he has many human weaknesses. He charms everyone with his flute playing. Magician is accompanied by Petrouchka, ballerina, and Arab, called Moor."

With his glasses pushed back on top of his head, Rostropovich reached matters of domestic conflict: "They all dance," he said, "Petrouchka falls in love with ballerina, Arab too. Scandal, jelousy. Ballerina faints. You know, normal household scene."

He touched on the pathos of the ballet, too. "Moor cuts off Petrouchka's head. Stravinsky wanted exact sound that symbolizes end of Petrouchka's life, bursting of his little red heart -- like pop of bubble. See -- on the snare drum. Petrouchka only a doll, sawdust fall down." At this Rostropovich brought from the orchestra a sudden shower of sawdustry sound. "Now," he concluded, "welcome to the Russian festival."

The performance was as entrancing as the illustrations, and, with the double bassoon burping in the lower regions, just as amusing. The orchestra was in razor-sharp condition, its soloists making eloquent, touching, subtle phrases in what is a kind of concerto for orchestra.

The vital piano part was played with fine incisiveness by the orchestra's new assistant conductor, Hugh Wolff.

Rostropovich opened the evening with a ravishing playing of the Rachmaninov Vocalise, phrasing it with exquisite rubatos, and eliciting a gorgeous sound. It was expressive in the manner of the grandest Stokowski.

Martha Argerich was the soloist in the big D Minor Piano Concerto of Rachmaninov. The Argentine pianist is an artist who could honestly be described, from last night's performance, as volcanic -- or cyclonic, or like a hurricane or tornado. She can be all over the piano in a flash, and there were times when her fingers dazzled with their huge cascades of sound.

But Rachmaninov was the most disciplined of pianists, and without that quality -- which was totally absent last night -- the work, for all its beauties, was often bombastic and bloated.