Ronald and Nancy Reagan danced most of the night and some of this morning, a sentimental end to the 70th birthday party that surprised him but nobody else. It had to be the worst kept secret in America.

Nancy Reagan begged the White House reporters not to let on they knew about the party, and yesterday morning, when the Reagans heard Tom Brokaw of NBC's "Today" show report the size of the Birthday party, she said to her husband: "My that Tom Brokaw certainly exaggerates."

"She pulled that off," Reagan told 100 friends after his wife's birthday toast in the East Room last night.

Looking around at the friends who ranged from editor William F. Buckley to Secretary of Defense Caspar Weinberger to singer Frank Sinatra, Reagan said there were a lot more there than he had been led to expect. It gave him a chance to point out the things Nancy has been doing with the White House, which brought up the subject of its size.

"In the old days I used to say 'Nancy, I'm home,' he said. "Now, when I get home I go to 132 rooms and holler 'Nancy!'"

Nancy Reagan, who let it be known she didn't want any other toasts except hers, said she was thankful for their friends but most of all, in the words of one guest, "thank goodness for my guy." At another point she said while it was hard to get used to being called first lady, she didn't mind at all calling him "the first man."

Most everyone danced with the First Couple but nobody danced more than they did. Nancy wore a white beaded gown that is one of her husband's favorites. The music was romantic and they even played "Nancy With the Laughing Face."

Despite the high-ceilinged formality that comes with the White House, one guest still described the evening as "just so relaxed . . . I haven't been to a party like that in years." Another guest called the evening "an absolute smash hit."

Nancy Reagan only wanted the president to know they were having "a small group of Washington friends" in for dinner in the family dining room. What he wasn't to know was that 100 friends were coming for lobster, veal, birthday cake and dancing.

"Please keep it a surprise," Nancy Reagan told reporters waiting for advance copies of the president's speech on the economy Thursday night. She said her gift would be a tree for their California ranch.

"Make sure it's a sturdy tree," ABC's Sam Donaldson advised her. "Carter took one to England to plant and two weeks later it died."

"You could have gone all night without saying that," she laughed.

Whether or not Reagan actually knew about the extent of the party is unclear, but like a young boy whose parents still believe he believes in Santa Claus, the president never let on.

"Surprise!" the guests yelled in the White House grand hall when Reagan came down at 7:45 p.m.

"He looked astounded by it all," insisted Nancy Reynolds, a close friend of the Reagans.

Guests in glittering evening gowns began arriving shortly after 7 p.m. Among the first was Reagan's brother Neil, who said he was two years and seven months older and "we use the same man to dye our hair."

Buckley arrived next, and was asked why he was attending Reagan's party since Reagan didn't attend his -- the 25th anniversary of the National Review -- in New York last year. "He was elected president," said Buckley, winking, "and I wasn't." As a gift for Reagan, Buckley said he was going to "repeal the 22nd amendment."

That's the amendment that limits the president to two terms in office.

Soon after Buckley made his way up the stairs to the grand hall, Undersecretary of State-designate William P. Clark arrived. Mr. Clark, Mr. Clark," reporters called out, "what are you giving President Reagan for his birthday?"

"That's a surprise," he replied.

"Mr. Clark, who's the prime minister of Zimbabwe?" another reporter called out, referring to Clark's inability to answer that question at his Senate confirmation hearing.

"That's a surprise, too," he said.

Gifts from other guests: presidential counselor Edwin Meese, deputy chief of staff Mike Deaver, Sen. Paul Laxalt (R-Nev.) and a few others pooled resources to give Reagan a Steuben glass sculpture for his desk that says "The Buckaroo Stops Here." Nancy Reynolds and the Alfred Bloomingdales also had gifts for his desk top, prompting Betsy Bloomingdale to remark, "Well, he's going to have a lot on his desk."

Others were more original. Charles Wick, co-chair of the inaugural events, announced that he had brought the president "a small country in Eastern Europe" and Reagan's daughter Maureen, who flew in without her father's knowledge, said her gift to him was "my plane ticket." Until the party started, she said she had "hidden" in a third-floor bedroom.

"It's not bad for an old house," she said.

Senate Majority Leader Howard Baker, asked if Americans are going to get a 10 percent tax cut, smiled weakly and said, "Sure, but not tonight."

The White House said the party was arranged and paid for by four couples: the Walter Annenbergs, the William Wilsons, the Earle Jorgensens and the Armand Deutsches. The plan was hatched over New Year's while they all spent the weekend at the Annenberg's Palm Springs, Calif., estate, a tradition on the groups's social calendar.

The couples drew up last night's guest list, and the Washington friends were clearly outnumbered by the Californians. The list was largely an honor roll of old friends, some from the Cabinet, Capitol Hill and the White House. It led to some interesting hierarchical oddities.

Secretary of State Alexander Haig for instance, was left off. But Clark, a longtime Claifornia friend was on.

Sen. Mark Hatfield (R-Ore.), chairman of the Inaugural Day ceremonies on Capitol Hill, was missing, but Abker, Laxalt, Sen. John Warner (R-Va.) and Rep. Robert Michel (R-Ill.) were there.

From the White House, Press Secretary Jim Brady was left off the list, but political director Lyn Nofziger was on. Chief of staff James Baker, Deaver, Meese and Peter McCoy, Nancy Reagan's staff director, were on.

From Hollywood: Frank Sinatra, Jimmy Stewart, George Murphy and Irene Dunne, who sat next to Reagan at dinner.

There was also a large part of the California crowd that seemed to be everywhere during the inaugural weekend: the Henry Salvatoris, the Justin Darts, the Holmes Tuttles . . ..

A lot of them arrived in town Thursday in time to catch the president's act on nationwide television. They watched him over dinner at the Jefferson Hotel, rating his economics speech as a good one. "It was a totally partisan crowd," said one guest afterward.

Last night, after guests had assembled in the grand hall, Reagan came down from the family quarters by elevator in a tuxedo, believing he was about to have the promised quiet dinner. (Asking him to wear a tuxedo wouldn't give it away, Nancy Reagan said, because the dinner, small or not, was to be black tie.)

After the surprise, guests sang "Happy Birthday, Ronald," then had cocktails in the Red, Green and Blue rooms then dined at tables covered with white organdy and green moire cloths. There were tulips, lilies, jonquils and lighted ficus trees. The effect was to be that of a spring garden. Guests ate from the FDR, Wilson and Johnson china services, which were switched with the courses.

The cake, vanilla sponge soaked in raspberry brandy and topped with a jumping white horse, was Reagan's second of the day. There were actually 11 cakes -- one for each of the ten tables, plus one large one. Chefs in tall white hats carried them in.

In the morning, congressional leaders had given him a four-foot-tall yellow cake topped with three toy elephants. That was in the Oval Office; Nancy Reagan had to be boosted up to light the candle.

By midnight at the second birthday party, when the Reagns showed no signs of going upstairs, guests began to slip out. "Nobody wanted to go before the president left," said one guest. "You felt like you ought to go, but you wanted to stay."

The Reagans were still dancing.