Forget about the cruise missile tests. Forget about the NATO alliance. The main news at the home of the Canadian ambassador last night (although everyone was much too with-it to gawk) was that Canadian Prime Minister Pierre Elliott Trudeau brought Margot Kidder, the actress.

"He was very encouraging with my work in the nuclear freeze movement," said Kidder, the Canadian-born actress who played Lois Lane in the movie "Superman." "We corresponded and then we met." She had on a tight-fitting black dress and very pale lipstick. Trudeau wore a tuxedo.

The embassy people said they didn't see anything to get excited about. "He's a single man," said Patrick Gossage, the press spokesman. "He likes beautiful women. You ought to come to Ottawa. Nobody pays any attention to Trudeau's dates. He just dates."

The prime minister, who wore a red rose in his lapel and was carefully kept away from reporters, is in town for talks with Ronald Reagan and George Bush. Last night he watched the president's speech on Central America and observed to a small group gathered around him: "I was impressed. It was the first time I've seen him on television." Today he's having lunch with the president and is supposed to talk about the upcoming Williamsburg economic summit and other weighty matters of state.

Last night it was generally dinner party babble, although babble by an impressive list of movie stars and directors, columnists, Democrats, Republicans, corporate executives, diplomats and TV people. A large number of the 80 guests who came to the ambassador's home on Rock Creek Drive were Canadian-born or had Canadian connections. There was almost no riffraff on the guest list.

"You can say I was the only man here who has not been mentioned as a successor to Paul Volcker," said economist John Kenneth Galbraith, referring to the Federal Reserve Board chief whose employment prospects appear to be down with the interest rates.

Well, would Galbraith like to be Fed chief?

"Anything involved in that job is a consummate bore," he said.

Volcker, meanwhile, was out on the terrace that overlooked the pool, all lit up in the warm April night. Canadian Ambassador Allan Gotlieb and his wife, Sondra, live well at their official residence here in Washington. Even the tulips out front have come up symmetrically.

So is Volcker going to stay in his job?

"That's a delicate question," he said.

Would he like to stay?

"That's a good question," he said, staring thoughtfully into his drink. This was followed by a very long pause, which was at last interrupted by Robert Strauss, the former chairman of the Democratic Party.

"I just want to thank you for getting me that cheap loan," Strauss said to Volcker, laughing heartily.

Volcker seemed to revive. "Yeah, let's get a scandal going," he said, sighing.

While this was occurring, White House deputy chief of staff Michael Deaver and undersecretary of state Lawrence Eagleburger were over in a corner, enmeshed in a conversation that looked like a pretty good one. Beirut? Watt? Women? Lunch?

It was lunch. They're going to have it soon.

Ambassador Gotlieb was concerned that the two weren't mingling enough. "Lots of interesting folk in there," he said, pointing toward the living room. "Movie stars."

"Movie stars?" said Deaver. "How about dinner?"

The "interesting folk" included:

Ethel Kennedy; director Alan Pakula ("Sophie's Choice"); actors Donald Sutherland and Christopher Plummer; Charles Bronfman, chairman of the board of Seagram Co.; producer Norman Jewison ("Best Friends"); U.S. protocol chief Selwa Roosevelt; Attorney General William French Smith; former defense secretary James Schlesinger; ABC's Barbara Walters; Roone Arledge, the president of ABC News; and Mortimer Zuckerman, owner of the Atlantic Monthly. He was born in Montreal, and he came dateless.

"I am actually by myself," he said. "I'm with the nation."

Sutherland, who is also Canadian-born, has found a home in this country, too. "I live here in Los Angeles," he said.

But this is Washington, somebody said.

"What time is it?" he asked, looking puzzled. He had blond hair that curled over his starched collar and he kept one hand inside his dinner jacket, Napoleon-style. He had just arrived at the party and was milling around in the lobby with a few other guests. "How do you think it's going so far--on a one-to-ten scale?" he mused.

Not too long after that, Christopher Plummer, who was very tan, was asked if he were at all intimidated by all the powerful and presumably smart politicians who surrounded the actors. "Heavens no," he said. "Usually they're intimidated by us."

An elbow or so away was Strauss. This time he was talking to the attorney general, who wanted to know all about Democratic presidential candidates John Glenn and Walter Mondale. It was interesting to see that Smith was there, particularly since the Justice Department recently labeled as "political propaganda" three Canadian films about acid rain and nuclear war. One of them won an Academy Award. In her acceptance speech, the recipient sarcastically thanked the Justice Deparment.

"Well, we like to please," said Smith.