It was back-to-party night at the opening of the National Symphony Orchestra. Lots of champagne, lots of kissing (mostly by Mstislav Rostropovich) and lots of people in the Atrium of the Kennedy Center.

Everyone was game:

Patrick Hayes, head of the Washington Performing Arts Society, gushed, "All the excitement! A new season! Summer's over! Time to work!" Then he ran off into the black-tie-clad, boisterous crowd.

Frank Hodsoll, who appears slated to become the new chairman of the National Endowment for the Arts, is still in the midst of his Washington social-orientation week. "I can see there are too many parties based on the last three days," he said cheerfully, taking it all in stride.

And from Hugh Wolff, who played the piano with the symphony last night and who is also Exxon guest conductor: "Well, I'm beginning to feel used to it," he said, grinning at the swirl of guests around him.

But nobody could outdo maestro Rostropovich. Last night he sent flowers and handwritten notes to every woman on the staff, the orchestra and the chorus. He also lost 12 pounds in four days.

"He came back from France, and his buttons were popping off his jacket," said Nadia Efremov, his translator. "I laughed at him. He was incensed."

So by last night he was in fine shape, loudly bestowing kisses upon everybody who came to greet him.

"I never believed you could play Haydn so well!" exclaimed one woman with emerald earrings. Smack! went Slava with a kiss.

The guests who poured several hundred thick past champagne and fruit and desserts following the concert included a few Reagan administration figures and a lot of arts administration figures, plus the members of the presidential task force who are scheduled to meet here today.

"Weinberger's here," said one guest, referring to the secretary of defense. "He's going to a lot of things these days. He's really enjoying life."

Rep. Sidney Yates (D-Ill.), a staunch arts supporter, sauntered through the party greeted by every fifth person, made a complete lap of the room, and paused to have his photograph taken with Patrick Hayes and NSO board member Elaine Silverstein. Then he waved goodbye excitedly. He was there maybe five minutes.

Martin Feinstein, former director of the symphony, was there and so was current National Endowment for the Arts chairman Livingston Biddle. "This party is usually for subscribers," said board member Austin Kiplinger, "and members of the orchestra and special friends. The family. Well, the extended family."

Last night it extended to many corporate executives who have given to the symphony or who might be potential donors.

"Yes, the guest list was done with malice aforethought," said Gerson Nordlinger, a NSO board member, smiling.