What do five members of the Supreme Soviet -- the equivalent of the U.S. Congress -- talk about during a four-day field trip to Washington?

"They talk about SALT, they talk about everything," said Soviet Ambassador Anatoly Dobrynin, who gave a small buffet dinner in their honor last night at the Soviet Embassy. "What is the topic of the day? Iran . . . Europe . . . Asia . . . SALT . . . trade . . . agriculture. We like SALT. It's up to you now."

The visit of the delegates (who go on to St. Louis and Denver later) is part of a regular exchange of American and Russian government officials that has been going on for the past few years.

Seventeen members of the House of Representatives, led by Rep. John Brademas (D-Ind.), traveled throughout the Soviet Union last April, meeting with some Supreme Soviet officials and Andrei Gromyko (but not Leonid Brezhnev) and seeing dancing classes, churches and the opera. Some of those members of Congress were guests last night.

The leader of the delegation from the Supreme Soviet is Sergei Medunov, a sober-faced man who wore a dark suit with a gold star hanging from a ribbon -- his country's highest award, he said. It was awarded for his part in inspiring the people of the Kuban region of the North Caucasus (where he is first secretary of the Communist Party) toward higher agricultural production.

The delegation met yesterday with members of the Committee on Foreign Relations. "We talked about furthering mutual understanding between our peoples," said Medunov through an interpreter. "We discussed questions of ratification of SALT by the Senate and the Supreme Soviet. In the Supreme Soviet there won't by any obstacles toward ratifying SALT."

And in America?

"My-optimisty," he replied. (We are optimistic.)

Said one American congressman of his trip to the Soviet Union, "I'd get back from events and one of my colleagues would say, 'Don't let them fatten you up.' I said, 'Fatten me up? For what?'"

"We worked very hard there," said Brademas. The embassy showed a Soviet-made film on the trip last night. "They were very hospitable. They want SALT ratified and trade extended."

"I come away from the Soviet Union with a feeling a strong belief that these people want peace very much," said Rep. Ron Dellums (D-Calif.) "For us, war is an abstraction -- it happens in other countries. But they lost 20 million people in World War II. The Soviets want peace even more than we do, I think. But it was unsettling to sit there and watch two countries who still have difficulty understanding each other. Both of us have relations rooted in fear and mistrust."

Dobrynin, who jauntily breezed through the party last night, turns 60 today. He will celebrate with a small party.

"You know Lenin was very modest," said embassy attache Eugene Peryshkin. "He never celebrated birthdays."