The summer night didn't fool anyone. With the Reagans back in the White House after a 25-day vacation, and Congress back on the Hill, Washington is once again in the business of government. And, appropriately, the fall social season was launched last night.

Marking the end of a 35-year hiatus in formal visits by Swedish leaders, President Reagan and his wife Nancy welcomed Swedish Prime Minister Ingvar Carlsson and his wife Ingrid to a White House dinner. The evening proved to be a celebration of U.S.-Swedish friendship.

"Our countries have been friends for as long as the United States has been a country," Reagan said in his toast, overlooking perhaps the long chill that began to warm when Carlsson succeeded the assassinated Olof Palme 18 months ago. For at least a year in the early 1970s the United States, angered by Sweden's sharp criticism of the Vietnam war, refused to accept Sweden's ambassador.

Carlsson, for his part, extolled the "opulent beauty of the Washington summer" and announced that he had been participating in "intense and interesting talks" with American leaders.

Moving on to specific concerns confronting the two nations, the Swedish leader said, "I understand you are near an historic breakthrough in the endeavor to start dismantling nuclear weapons. The fact that such an agreement is near ... is a symbol of hope for all mankind."

After the toasts, the president fielded questions from reporters in the Blue Room. Asked whether he was angry at Supreme Court Justice Thurgood Marshall's recent statement to WUSA-TV that he rated the president at "the bottom" of U.S. presidents in terms of racial justice, Reagan offered a trademark smile.

"A young fellow like me is not going to get mad at an old fellow like him," he said. Reagan is 76; Marshall is 79.

Pressed for further comment, the president turned serious. "I hope he will be informed that isn't my record, not only in the administration but also as governor of California," he said. "In fact, I was raised in a household in which the greatest sin was prejudice. I just wish he had known that.

"From boyhood on, I have been on the side of civil rights and no discrimination and I am just sorry that he is not aware of that."

On Gary Hart's "Nightline" appearance, in which the former presidential candidate once again criticized press probing of politicians' personal lives, Reagan said, "I suppose if you're in public life you have to accept that." Then he added that it hadn't been too many years since reporters hesitated to quote a president without his permission.

And would he like that privilege today?

"It might be nice," he said.

Asked what message he would have for Pope John Paul II, whom he meets in Miami today, Reagan responded, "I'm hoping the pope will have a message for me."

Would the president, the son of an Irishman, kiss the papal ring?

"I don't want him to find out I'm Protestant."

Asked when Soviet leader Mikhail Gorbachev will visit the United States for a long-awaited third summit, Reagan said it could be in either October or November but that "we believe late November is probably best."

Asked later about Senate Minority Leader Robert J. Dole's "little three-day invasion" of Nicaragua -- his remark to an interviewer that many Central Americans would welcome a U.S. invasion -- Reagan said, "I was very proud of him. He held his own." A Dole spokesman has since disclaimed the remark.

As the evening began, the Reagans awaited the Carlssons on the North Portico, while a modest crowd along Pennsylvania Avenue watched the Carlsson motorcade disappear through White House gates. Nancy Reagan wore a figure-hugging orange and white gown by one of her favorite designers, Galanos. Ingrid Carlsson was dressed in a two-piece powder blue and white gown.

Last night's guest list was as always a judicious mix of Washington, Hollywood and points between, of cultural luster and political payback.

Novelist Walker Percy and his wife Mary Bernice rubbed elbows with M. Carl Holman, president of the National Urban Coalition; Librarian of Congress Daniel J. Boorstin -- whose presence suggested some deference to Ingrid Carlsson, who works at the Stockholm City Library -- shared the list with leggy Harlem Globetrotter Lynette Woodard.

Arriving at the East Wing entrance, guests acknowledged a back-to-school atmosphere in the still-balmy air. "I think the fall is going to be a good one," said a tanned Treasury Secretary James A. Baker III.

Deputy Chief of Staff Kenneth M. Duberstein concurred. "We're going to go flat out on a lot of major priorities," he said, suggesting that it will be a busy fall.

"I'm hoping for Thanksgiving," said Sen. Daniel J. Evans (R-Wash.), when asked how long he thought Congress would have to stay in session, "but I'm planning for Christmas."

Said Rep. Bill McCollum (R-Fla.), "It's pretty good to be back. The fall is always nicer than the summer in D.C. It was awful hot here this summer -- for me especially." That was a probable allusion to McCollum's service on the Iran-contra investigative committees, on which he was considered one of the questioners friendliest to administration witnesses -- along with Sen. Orrin G. Hatch (R-Utah), who was also a guest at last night's dinner.

Songwriter Marvin Hamlisch, who has entertained at earlier White House dinners, came alone and appeared to be savoring his status: "I don't have to play. It's fantastic."

That honor fell to singer Marilyn Horne, who performed 21 years ago at Reagan's first inauguration as governor of California. The six songs she sang ranged from Rossini arias to "Jeannie With the Light Brown Hair."

Hamlisch later kissed her and gushed, "Marilyn, marvelous -- 'I Dream of Jeannie' was to die."

Horne rated her own performance "okay," but added, "It was an enormous hoot to stand there in front of all those faces that I know."

Candice Bergen, who is of Swedish descent, came with her husband, director Louis Malle, whose new film "Au Revoir les Enfants" -- which stars Bergen -- won the Golden Lion Award in Venice yesterday. The actress, striking in a strapless navy blue gown by Chanel, was asked whether she was looking forward to the fall premiere of her new TV movie, which was based on the memoirs of "Mayflower Madam" Sydney Biddle Barrows. "I am -- at least I think I am," she said. On motherhood -- her daughter Chloe is not quite 2 -- she was more certain: "It's wonderful."

Middleweight boxing champion Sugar Ray Leonard, beautiful in a custom-designed Valentino tuxedo, caused a big stir among the sizable Swedish press corps that accompanied the prime minister, but could talk of little other than his dinner partner: "I had a chance to meet Candice Bergen. She's fabulous!"

White House Chief of Staff Howard Baker was not so lucky: After the toasts but before Horne performed, he was seen slipping into his limousine for the drive home. "Don't you like music?" someone asked.

"I love music," Baker said. "But I like sleep, too." What he didn't know was that Candice Bergen hoped above all to meet him, and spent the evening scanning the crowd in vain. "I know he's short," said the willowy actress, "so he might be hard to spot."

The menu included Chesapeake Bay crab and chicken with wild rice and walnuts and lemon sorbet. The food was pronounced fine by several guests, although Malle made light -- as only a Frenchman can -- of the Domaine Chandon Blanc de Noirs served after dinner. "This purports to be champagne," he sniffed.

A more quizzical note was struck by Anders Ferm, Swedish ambassador to the United Nations, who arrived after dinner. "I'm just an ignorant Swede," he said, collaring a reporter in the Cross Hall connecting the State Rooms. "Are they all like this?" he asked, meaning the party. Gesturing at the surrounding presidential portraits, he said, "I think the paintings are ugly. But the music is nice."

Earlier yesterday, Carlsson was honored at a State Department luncheon hosted by Secretary of State George Shultz where he was publicly reminded that there is one bilateral problem between Sweden and the United States.

Apples.

Shultz, to the delight of his 200 guests, sprang it on Carlsson during the toasts that followed the shellfish pa~te', beef tenderloin and apple brown betty dessert.

"I hoped you noticed that I'm not going to leave it to negotiations to make sure the United States makes a point of one trade issue," Shultz said, grinning at Carlsson, who sat across the room with his hostess, Helena Shultz. "We think our American apples are pretty good."

So good, in fact, that Shultz said he told his staff to put apples on the menu for the luncheon. The Swedes have established seasonal restrictions limiting the import of apples and pears. "Let's persuade the prime minister, by the taste, that American apples should be fully available to the Swedish people."

Gracefully substituting apples for crow, Carlsson, when his turn to toast came, noted that only the day before he had categorically told journalists there weren't any bilateral problems between the United States and Sweden.

"Now," he said, feigning sheepishness, "I understand we have -- we have apples."

The high-level teasing was symbolic of the efforts by both countries to let bygones be bygones.

Shultz told his guests that "the difficult years are behind us" and that "the substance of our relationship is of importance to both of us."

At a morning meeting with the president, Carlsson had stressed Swedish disagreement with U.S. policy in Central America and with the so-called Strategic Defense Initiative, the administration's program to develop a space-based antimissile defense.

But at lunch, Carlsson said, "It's natural for us sometimes to judge international affairs differently, but we recognize that such difference of opinions should not allow us to disturb our bilateral relations."

Among the others at Shultz's table was Swedish tennis great Bjorn Borg, who as a member of the Swedish Tourist Board is a part of Carlsson's official entourage. Borg was scheduled to team up with Vice President George Bush this morning in a tennis foursome rounded out by Swedish Ambassador Wilhelm Wachtmeister and Vitas Gerulaitis.

At Deputy Secretary of State John C. Whitehead's table, veteran news anchor Roger Mudd, after some delay followed by lengthy debate among a growing number of waiters, was served a heaping plate of carrots, asparagus and potatoes, but no meat.

"Do I look like a vegetarian?" asked Mudd, eyeing the beef tenderloin on the plates around him.

Whether he looked like one or not, he was sitting where a vegetarian was supposed to sit, and as far as the waiters were concerned, orders were orders.

Mudd smiled patiently but persevered in his pursuit of meat. This prompted yet another huddle, followed by the plate of vegetables being carried away, leaving Mudd with nothing but the sympathy of his companions.

"That's always been my impression of Washington -- it takes a committee to do anything here," murmured an out-of-town guest, who asked to remain anonymous lest she never be invited back.

Eventually Mudd was served and even managed to catch up with the apple brown betty before it was all gone.