President Reagan dropped in on his favorite think tank last night and while he didn't burden his audience with all that much to think about -- "I knew we had a recovery when they stopped calling it Reaganomics," he said at one point -- he did leave them laughing.

"What's nice about him," said Dennis Bark, a senior fellow of the Hoover Institution at Stanford University, which hosted the party at the Corcoran Gallery of Art, "is that he makes people stop worrying about whether everything is all right and gets them to relax."

About 600 people managed to stop worrying last night. They relaxed by drifting between buffet tables of roast beef, crab claws and oysters on the half shell.

Some came from California for the meeting of the institution's 40-member board of overseers. Others came from around town because the president did. The president came from across the street because he's an honorary fellow.

The institution made him its first fellow in 1975, the same year he gave it his gubernatorial papers. His presidential papers come next, when the Reagan presidential library is built at Stanford.

Guests shivered in the freezing night air as they lined up outside, waiting their turn to enter. They had just made it when the Reagans, who didn't have that trouble, walked onto the platform with the institution's director, W. Glenn Campbell; his wife, Rita Ricardo-Campbell, and cohost Emil Mosbacher Jr. with his wife Pat.

Nancy Reagan's black Adolfo dinner suit, banded with elaborate gold embroidery, drew an appraising glance from one guest. "That," the woman knowingly told her companion, "is a Valentino."

Stanford alumni included Supreme Court Justice Sandra Day O'Connor and Allan Hoover, son of Hoover Institution founder and former president Herbert Hoover.

Said Allan Hoover when asked if he saw any resemblance between Reagan and his father: "They both speak English."

Hoover said his father got the idea for the institution "when he was feeding all of western Europe and saw a lot of documents being destroyed. He thought they ought to be saved and said, 'You collect all this and I'll pay for it after the war.' I don't have the remotest idea what he paid but it's priceless now. We have stuff there the Russians have to come over to see because they don't have it."

Other people talked about job swapping, reducing the deficit and negotiating with the Russians. The president didn't get into specifics about any of those topics.

"After you pass 40, you know," he said, "you suffer from lumbago and the tendency to tell the same stories."

If Defense Secretary Caspar Weinberger, Secretary of State George Shultz, CIA Director William Casey and Interior Secretary William Clark had already heard the ones about a farmer and some politicians, they laughed anyway.