What does a senator do on the weekend in Washington? Often, the family lives in another state, and it's not always possible to fly home, so a senator has to be an expert in filling free time.

Senator Barry Goldwater (R-Arizona) speaks with the serenity and assuredness of a self-directed person who has all the time in the world because he knows how to use it. He knows what he feels like doing and he does it; whether it's piloting a plane, operating a ham radio, taking photographs, reading mysteries or playing golf.

"In this job in the U.S. Senate," he says, "your brain gets over-exercised, you can't function. When that happens, then just doing something else -- flying, fiddling around with electronic gear, working on the car -- refreshes my mind. Then I come back Monday ready to go."

After 30 years in Washington, he doesn't feel obliged to go to parties. "I don't accept invitations generally, because my wife's not here," Goldwater said on the phone the other day, "and I know there's nothing worse than an extra man. And, frankly, the extra men never know what they're going to wind up with.

"I have a few friends I visit and that's all the social life I need," he said. He often spends the weekend with friends at a farm near St. Michaels, Maryland. Goldwater is the perfect houseguest: He does electrical work, carpentering and plumbing. "I keep the house running," he says. They call him the head man.

One recent weekend when an aviation meeting spilled over into Saturday, Goldwater spent the time instead at home in his apartment, recording old Dixieland jazz records on cassettes. Along with his other interests, he's a jazz buff.

If he eats dinner at a restaurant, it may be at Nora or the Jockey Club. "I don't go out too much," he said. "I do my own cooking, which is confined of course to things that are difficult like frozen foods, hamburgers and steaks." He makes chili, too.

"I don't think I've seen a moving picture in a theater since 'Silver Streak.' I just can't put up with people eating popcorn, chewing gum and everything else they do in theaters today. I'd just as soon stay home." Like most people, Goldwater watches movies on television. He favors westerns and owns a few copies -- "High Noon" and "She Wore a Yellow Ribbon" -- which he plays on a Betamax.

He may avoid large parties, but several times a week he speaks to people all over the world on his ham radio. "I have a little station right behind where I'm sitting now in my office talking to you," he says, and one in his apartment as well. Ham radio operators don't use last names, just call signs. Goldwater's is K-7UGA.

"We're not supposed to talk politics," he says. "Mostly we talk about our own equipment, what's going on in the amateur field."

For exercise, he tries to swim a mile a day, either in the Senate pool or in friends' pools: "I don't make it, but I try," he says. "You've got to get some exercise in this job." He's been to Rehoboth only once, and Ocean City once. "It's just too doggone far to go," he says.

Goldwater quit golf four years ago because of a bad hip, but since a replacement operation cleared that up, he plans to hit the greens again someday soon. He loves competitive sports, he says, but: "I'm coming on 74 years old. The reason I'm in bad shape is I played too much competitive sports when I was young," riding in rodeos, playing football and basketball, running track.

When he really wants to relax, he will borrow or rent an airplane or helicopter, "and just go up and fool around. I get the same satisfaction from that that some people get from bridge -- which I don't play -- or chess -- which I don't play."

With all these hobbies, one might well ask how he manages to fit everything in on the weekend. Goldwater does have a little trick: "I never get out of bed later than 5 o'clock," he says.

If he wakes up early, say at 4, he reads until the newspaper arrives. Besides mysteries, he enjoys history; at the moment, he's forging his way through a paper on the meaning of power. Next on his list is "a big, thick book called China, which I'm going to get through somehow.

"I think that more people waste their weekends by just staying home and not doing things," he says. "That's nice, too, but I don't know of another way to start growing older than not doing anything."