"I was talking to Mr. Brezhnev in August," said millionaire art collector Armand Hammer last night at a dinner and cocktail reception at the National Gallery of Art marking the opening of the Hermitage art collection. "And he asked me, 'What can I do for you now?'

"I asked him, 'Let us have the Leonardo.' Some of his assistants shook their heads and said, 'Nyet,' but Mr. Brezhnev said, 'da.'

"I asked for the Leonardo and a small group of paintings from the period-we got a list from Carter Brown-and they gave us almost everything we aksed for, except a few pieces whose condition wouldn't allow them to be moved."

Hammer's straightforward explanation contrasted with the more convoluted theories being put forward by some art insiders.

"First, they were going to send over more than 30 paintings," said one New York expert. "Then came the Chinese accords and the whole agreement was off. Finally, they sent 11-but nobody knows why they sent these 11, and it was a real challenge trying to make sense out of it as a unified exhibit."

Art gossip and amateur Kremlinology dominted the cocktail party conversation at the preview of the collection, "The Legacy of Leonardo: Italian Renaissance Paintings From Leningrad."

At the dinner, J. Carter Brown led a champagne toast to Leonid Brezhnev and Soviet Minister of Culture Petr Demichev led a toast to Jimmy Carte. Despite these toasts, the heroes of the evening were Hammer and Everett Fahy, author of the exhibition catalogue, who made thematic sense out of it in what one admirer called "a brilliant tour de force."

The highlights of the exhibit were two paintings dated nearly a century apart: Leonardo's "Madonna and Child" (also known as the Benois Madonna after a former owner) and Titian's 'St. Mary Magdalen in Penitence."

After finishing its stay at the National Gallery, the exhibit from Leningrad's Hermitage Museum will go on to the Los Angeles County Museum and then to the Knoedler Gallery in New York. In exchange, a collection of American-owned paintings of similar vintage will tour four Russian museums.

"Some of the art people in New York are furious that this exhibit is going to the Knoedler," said one insider of the New York art scene during the cocktail reception. "They think it should have gone to the Metropolitan-but of course Armand Hammer owns the Knoedler, and these paintings wouldn't have come over without him."

One of the guests at the preview was Peter Ustinov, whose family had once owned the Leonardo (Benois was the name of grandmother's uncle). "They were caviar tinners," the actor said, "and they bought the painting from some traveling Italian entertainers who couldn't pay their bills. It didn't stay in the family long - just long enough to acquire a sobriquet." CAPTION: Picture, From left, Petr Demichev, Armand Hammer and Sen. Charles Percy; by James A. Parcell-The Washington Post