Do you know how to say glasnost?

Mister Rogers, the star of children's television, was at the Soviet Embassy yesterday, singing "won't you be my neighbor" to a bunch of Russian and American kids who showed no interest whatsoever in singing along with him.

"It's a beautiful day in the neighborhood," he sang, while diplomats and journalists hovered and smiled and the children cast a cool eye on the whole business.

Then again, they showed no interest in singing along with Tatiana Vedeneeva, either.

She's the star of "Good Night, Little Ones," a children's television show in the Soviet Union, and she has that same way as Fred Rogers of smiling an infinitely congratulatory smile at children no matter what they do or don't do, a smile designed to end conversations with children on a nice note, the way "Let's have lunch sometime" does among adults in Washington.

Do you know how to say cultural exchange? Do you know how to say photo opportunity?

Strobes flashed, cameras rolled, the huge foam-cozied proboscises of the television crews probed into conversations around the room.

"It's a special Soviet-American meeting," said Soviet Ambassador Yuri Dubinin. "Such a meeting ... will be transformed into an enormous meeting through television."

Then a tape was run of Mister Rogers visiting Tatiana in Moscow.

"It's from the time I went to visit Tatiana in her studio," he told the kids at the press conference -- Soviet Embassy kids and kids from the Arlington Day School. "I thought you'd like to know what a studio in Russia looks like."

What a strange idea! But who knows children better than Mister Rogers?

On the tape he plays the piano and introduces one of his hand puppets to Tatiana's pig-puppet Stepashka, saying, "Would you like to meet a puppet from America?"

There are differences between children's television in the two countries, apparently.

Mister Rogers wasn't wearing his cardigan; he was wearing a coat and tie, but somehow everything he wears looks like a cardigan, nice and homey. Tatiana, on the other hand, was wearing a black leather skirt, three-inch heels, blue eye shadow and a James-Bond-call-your-office see-through blouse over lace underclothes. In short, she is not the sort of children's television figure we're used to, compared, say, with the progressive androgynous image established years ago by Maria on Sesame Street, or the megacuteness of Shari Lewis.

Then again, she does come from a country whose leader recently called for women to leave the workplace and "return to their purely womanly mission."

Is she pretty? a couple of children were asked.

"Real pretty," said Serena McGee, 3, of Arlington.

"B-i-i-i-g pretty," said Jose Tellez, also 3 and of Arlington.

"We all hope in the Soviet Union that this visit with Gorbachev will bring good and great results -- I'm sure that nobody in the United States or the Soviet Union wants war," Tatiana said through an interpreter.

Secretary Gorbachev is to arrive here on Dec. 7 for a summit meeting with President Reagan. Did that have anything to do with Stepashka the Pig and what a beautiful day it was in the neighborhood?

Ambassador Dubinin said, "I am sure that all of these things are interconnected."

Asked whether America or the Soviet Union would get the most benefit out of this kid-vid exchange, Mister Rogers said, "I never thought of it that way -- I think we all stand to gain a great deal."

Tatiana will visit Mister Rogers' studio in Pittsburgh, and the shows they tape will air in March. Mister Rogers will appear on "Good Night, Little Ones" later this year.

Do you know how to say gain a great deal?