Sunday morning, Beverly Hills. The lawn, thick and moist as a golf course at dawn, squishes under Peter McCoy's sneakers as he pads past his pool, past his apricot trees, past the garage that holds the Mercedes and gleaming Model A Ford. Frank Sinatra's house is above on a ledge of Coldwater Canyon, as much a part of these hills as the coyote and deer.McCoy, a president of Sotheby Parke Bernet, on leave to be Nancy Reagan's new staff director, is en route to his greenhouse. "I have to pack up my orchids," he sighs. He strokes a leaf tenderly. "But they'll probably all die."

Outside, within view of the orchids, is a neat patch of soil. "We used to have a little vegetable garden out there," he says."And I don't even have a yard in my new place." It is warm and humid, like Washington in late September. The hills are hushed.

He walks, as if on a continuing farewell tour, toward the garage and Model A. A pang. "My old car has to go into storage. I won't have a garage for it, that's the problem." A cat naps, as is her custom, on the convertible roof of his wife's Mercedes. There's a sheet of cardboard under her to keep fur off the fabric.

"Sweet face," says McCoy, scratching her chin. She looks as content as he used to be, before Nancy Reagan summoned him to Washington. "It's a big adjustment," says McCoy, feet now up on a glass table in his den. He's wearing khakis and a red checked button-down, waiting for the housekeeper to bring coffee in the Wedgwood bone china. "Aah, jeeze," he continues, "what are we getting into?" The Auctioneer Unseated

Four years of running Nancy Reagan's staff in a city with a life style far different from his art world in Beverly Hills. No more auctions, the twice-a-week events where he sometimes sold to Ryan O'Neal or Farrah Fawcett. No more Los Angeles cocktail parties, like the one he went to with his wife Kacey on the night before this Sunday morning. Candice Bergen, Johnny Carson and Cary Grant came.

"Candy Bergen had a party for her mother to meet that guy she married," he explains. "See, Candy grew up with Kacey, and I've known her for 20 years." The fire in his fireplace, built because it is chilly by California standards, glows under an oil painting by Julien DePre. The coffee has come, with real cream. On the table are 18 change-of-address forms for the household magazines, Town & Country, Gourmet, Organic Gardening, The American Orchid Society Bulletin . . .

They'll be sent to his new house in Spring Valley, recently bought from Energy Secretary Charles Duncan. It cost so much McCoy looks a little sick when he talks about it. So he'll rent out his home in Beverly Hills.

"I don't know what to expect in Washington," he says. "It's like going into a business you don't know anything about . . . Kacey keeps saying that moving is the second most traumatic experience in a person's life. You couple that with children in a new school, me putting in long hours, not 100 percent sure of how my situation will run -- and God, it is nerve-wracking."

On Jan. 20, McCoy becomes deputy assistant to the president as well as East Wing staff director, the job held in the Carter administration by Kit Dobelle. She is the wife of the former mayor from small-town Pittsfield, Mass. and is as different from McCoy as Reagan's administration will be from Carter's. McCoy is West L.A. society, a man who moves easily from dinner with Sinatra at New York's 21 to sharing a house, as he did during the campaign last fall, with Reagan aides Mike Deaver and Joe Canzeri in Middleburg, Va. He posted a shower schedule on the bathroom door.

"It was like a frat house," laughs Canzeri. "He made the house rules. So we used to call him Mother -- Mother McCoy."

He is 39, the father of two, slender, slightly balding, delicate-looking in his checked red shirt, elegant-looking in a three-piece suit. The gray one he wore mid-December in Washington came with a silk tie and a dangling gold watch engraved with P. M. The One-Line Wonder

With Nancy Reagan, on the campaign trail McCoy "was kind of the chief of staff," Canzeri says, "doing everything from carrying her purse, to making the telephone calls, to advising her about what to say at events. He's funny and he's witty and he does the job without taking it or himself too seriously."

The two usually sat next to each other on campaign flights, chotling over stories in the newspapers. McCoy was always ready with a one-liner to lighten the tense moments, and in fact, it sometimes seems hard to get him to abandon the one-line mentality. Asked what the most challenging part of his new job will be, McCoy cheerily replies: "Finding a parking space."

Canzeri recalls the time at Eureka College, Reagan's alma mater in Eureka, Ill., when Reagan, Nancy Reagan, Reagan's former football coach, the college president, a priest and McCoy were standing in a small room, waiting for the candidate to speak in the gym. McCoy, just off the campaign flight, had collected eight miniature Scotch bottles for the staff ("Money was tight," says Cazneri.) McCoy had put the bottles in his briefcase, then left it accidentally unlatched. When everybody got up to leave the room, so did McCoy with his briefcase. The Scotch bottles came trumbling out, clinking noisily in full view on the floor. Reagan turned and raised an eyebrow.

"I should have said," says McCoy now, "'That's okay, governor, none of them broke.'"

McCoy's sense of humor is clearly a plus with Nancy Reagan, a woman who enjoys men, like her friend Jerry Zipkin, whose amuse and entertain. McCoy's well-bred affability, coupled with his ease at handling the day-to-day snags, made him a favorite on Nancy Reagan's campaign trail.

"She just sort of enjoyed bantering with him," says Coral Schmid, who traveled with them as Nancy Reagan's press secretary. "He has a way of smoothing things. She worries all the time, but Peter, outwardly, is very calm. He was a good balance for her. On the campaign, there were so many stops and cities -- so many times she would look at him and say, 'Peter, where are we going now?'"

McCoy also served as Nancy Reagan's frequent escort, a stand-in for her husband. On Labor Day weekend in New York, it was he who went with Nancy, Ron Jr. and Ron's soon-to-be-wife Doria to a performance of "42nd Street." "She's genuinely fond of him," says Schmid. "He's extremely cultured, well bred, and he takes care of all the niceties."

"He's not a big joke teller," says his wife, "but he's certainly amusing. Then he has these gardening moods when he sort of disappears into the wilderness."

This Sunday, McCoy is amusing.

Q: "You are Republican?"

A: "Oh yes, have been for a couple of weeks now."

He is also the son of novelist and screenwriter Horace McCoy, who wrote "They Shoot Horses, Don't They." His wife is 35, the granddaughter of Edward Doheny, one of California's early oil men. The two are popular in young establishment West Los Angeles, a crowd that is a generation younger than the Bloomingdales and Wicks who socialize with the Reagan's. The McCoy's don't go to their parties, but then Kacey's parents do.

It was at a party in 1975 that McCoy ran into Deaver, an old friend, and asked him what he could do for Reagan. Handle seat arrangements and logistics at the '76 convention, he was told. He did the same thing at the 1980 convention, and after the nomination, served as Nancy Reagan's chief aide and traveling companion. Sometimes he carried her luggage.

"I mean if there was no one to grab it," says McCoy, "I would grab it." Menagerie and the Fishbowl

The morning fire still glows, making the brown marble fireplace reflect the dancing light. In one corner of the den is a bar. In another a six-foot ficus shading a tiger skin rug. The walls are light green, the couch splashed with orange and yellow flowers. "I love pretty things," says McCoy.

A cockateel screeches from another room, but you can't hear Homer, a complacent pet rat. "He doesn't even know how to run, he's so fat and awful," says McCoy. Housekeeper nowhere in sight, he ducks out for more coffee.

He returns with a glass pot, pouring gently. "All of the silver is packed," he sighs. "My wife would have a fit if she knew I was serving out of this thing."

He's right. "Oh, we don't even have a coffe pot," says Kacey when she spots the offender on its wicker tray. "And look at this carpet." White, with a slight spot or two.

She has a full, pretty face. No makeup this morning, and thick brown hair pulled back with barrettes. She's been packing upstairs but now plops on the couch.

The subject of Washington as fish-bowl surfaces.

"Why will we be living in a fish-bowl?" asks McCoy.

"Oh, yes," says his wife. "Yes . . . here, it's a different visibility, it's old family. We've both lived here forever and our families have been here forever . . ." Mailroom to Management

Both of them grew up in Beverly Hills, McCoy left fatherless at 14. Instead of college he went to the mailroom of the William Morris Agency and in seven years, had worked his way up to become an agent in the theatrical and television department. He was an independent management consultant when Sotheby's chairman in London, Peter Wilson, hired him nine years ago to take over the small office in Los Angeles McCoy knew management skills as well as he knew the rich of Los Angeles.

He came to oversee a business that has 75 employes, a multimillion-dollar volume, and a receptive clientele among the growing West Coast art market. McCoy had never been in that kind of business before, but as Marvin Newman, an executive vice president at Sotheby's puts it, "He's a good administrator, he doesn't blow his top too often, and he mixes well with people."

"Every day was new and exciting," says McCoy of his time at Sotheby's. "You'd walk into a house and you'd find a hidden treasure or you'd see a fabulous masterpiece. Once on a trip to Santa Barbara, I was going to look at some woman's paintings. Well, they turned out to be Safeway supermarket reproductions, but her kitchen table was Chippendale. She didn't know that. She thought it was something to put the cat food on."

McCoy became an auctioneer by accident, one night when the regular man didn't show. Three years ago, he got $980,000 for George Caleb Bingham's "The Jolly Flatboatmen," a record at the time. The rest of his duties were administrative, the background that he plans to use at the White House.

So far, he's hired six people for a staff that eventually will have 18 and a $600,000 budget. His salary will be somewhere under the $56,000 that Kit Dobell received. He won't say much else about his plans for a job that oversees the first lady's day-to-day operations at the White House -- from invitations to state dinner seating to what to say in speehes.

He also won't say much about how he plans to coordinate the fist lady's East Wing with the president's West Wing, a union created by his title in both places. Some one surmised that McCoy as deputy assistant to Ronald Reagan and staff director to Nancy Reagan will assure that the West Wing has total control over the East Wing. In previous administrations, the two kingdoms have often clashed.

But again, no answer. "We really haven't worked that out yet," says McCoy. "It's too early to tell." The Road Not Taken

Sunday morning has turned to Sunday afternoon. The cockateel has stopped screeching, but the dog barks. McCoy pretends that the 18 change of-address forms for his magazine subscriptions are playing cards. He spreads them out in a fan, eying them, like a bad poker hand, a little sadly.

His wife, feet up on the couch, is still fretting about the move to Washington. "But," she says, "can you think of not doing it? And then saying to yourself, 'I wonder how it would have been?'"