Only their interpreters know exactly what they said, but Nancy Reagan's much-touted tea for Raisa Gorbachev went so well today that it lasted a half hour longer than anybody had planned.

In fact, the Soviet first lady was still there chatting and sipping Mrs. Reagan's favorite almond-flavored herbal tea when President Reagan came home from the first of his summit talks with Soviet leader Mikhail Gorbachev. Reagan slipped in the back door without meeting Mrs. Gorbachev, an introduction that came at a dinner the Gorbachevs gave for the Reagans tonight.

If there was a news blackout from Fleur d'Eau, where the two leaders are meeting, there was none at all from the Maison de Saussure, where the tea party summit was in progress.

Afterward, Mrs. Reagan told American reporters that she and her guest talked about "our husbands and the meeting and what we hoped would come out" of it. She said each invited the other to visit her country. "We both said we hoped we would be able to go," Mrs. Reagan said.

Asked to elaborate on what they hoped would come out of the summit, Mrs. Reagan said, "A better understanding . . . What her husband wanted, what my husband wanted is the same. A better understanding."

Standing in the foyer of the 18th-century residence as she talked to reporters, who included her son Ron Reagan, Nancy Reagan said she hoped she and Mrs. Gorbachev could help improve relations between their two countries.

"This is one of the things we talked about. Personal contact is always helpful," Mrs. Reagan said.

The American and European press covered Mrs. Gorbachev's arrival and departure at the tea, but there were no Soviet reporters present. Nor did Soviet television audiences see anything of Mrs. Gorbachev's day, which was chronicled by the rest of the press corps.

Mrs. Gorbachev reportedly showed a fine sense of humor all day, charming those she met. She began her day at the Clock and Watch Museum, where she praised especially two cases of 16th-century watches -- at one point saying in English, "They're beautiful" while television cameras rolled. At the city university library, where Soviet founding father Vladimir Lenin studied, she attentively examined a collection of Lenin memorabilia. As she left, she walked to a crowd of several hundred students chanting her name and told them, "The future belongs to young people. We must struggle for peace."

The tea began at 4 p.m., when Mrs. Gorbachev arrived in a Soviet limousine, part of a five-car motorcade that brought her to the Reagans' borrowed lakeside residence a few miles outside Geneva. Mrs. Reagan was waiting for her. So were photographers.

Mrs. Reagan seemed willing enough to pause for pictures, but after about a second or two of standing there in the icy late-afternoon air, Mrs. Gorbachev obviously had had her fill.

"It's enough, all right?" she said, looking first at the photographers and then at her hostess. "Yes," Mrs. Reagan replied, taking her by the arm and guiding her into the house. "She's cold. Aren't you?" Mrs. Reagan asked the photographers.

Inside a few minutes later there was a wave of what the White House calls "photo opportunities," as American and European photographers took turns photographing the two women as they sat side by side in the elegant green drawing room, across from a crackling fire.

"It's quite warm here. We can pose here for quite some time," Mrs. Gorbachev said in Russian, with an interpreter translating.

She had shed her light gray raincoat and multicolored scarf and was wearing a dressy black suit with a white blouse tied at the neck, opaque black stockings and high-heeled black shoes. From her ears dangled diamond earrings, and on the middle finger of her left hand a multidiamond ring.

Mrs. Reagan wore a turquoise and black checked jacket, high-necked and wide-shouldered, over a slim black skirt, with black pumps and sheer black stockings. She wore gold earrings.

Mrs. Gorbachev smiled but looked a little ill at ease for the first time in the day, sitting on the edge of her chair. Mrs. Reagan was more relaxed and seemed to be taking it all in with a bemused expression. Asked about it later, Mrs. Reagan said she thought "everything relaxed after a while" and that Mrs. Gorbachev was "a very nice lady."

She said she and Mrs. Gorbachev would resume their discussions when she goes to the Soviet mission for a reciprocal tea Wednesday.

At tonight's dinner in Villa Rose, the Soviet compound house where the Gorbachevs are staying, the two couples seemed to be getting along amiably. When one reporter asked if it could be assumed that the two leaders were getting along well, Gorbachev said through an interpreter, "Let's say that's your judgment. You have expressed your judgments."

Had they reached any agreements?

"We are working, we are continuing to work," Gorbachev said.

And did his remark earlier in the day to the Rev. Jesse Jackson that space weapons would hold the world hostage mean Reagan had not yet changed Gorbachev's mind? The Soviet leader had another ready reply.

"Keep that question," he said. "That is a question that should be explained at a press conference later."

When reporters asked Reagan how things were going, he answered, "We're still smiling."

For the wives, it was a resumption of the afternoon get-together. This time, though, Mrs. Reagan wore a shimmering gold, blue and red brocade two-piece outfit, with sheer black stockings and black satin shoes. Her jewelry included a sparkly bracelet.

Mrs. Gorbachev wore the same light-colored raincoat she had worn on arrival at Nancy Reagan's tea. Under it was a black knit dress with gold trim. She wore black stockings, gold platform shoes with ankle straps and the same diamond earrings.

The two were the only women at the dinner. Mrs. Reagan was seated at the table between Gorbachev and Soviet Deputy Foreign Minister Georgi Korneiko. Mrs. Gorbachev sat next to Reagan and Soviet Foreign Minister Eduard Shevardnadze.

Others among the Soviet and American officials were White House chief of staff Donald T. Regan, national security adviser Robert C. McFarlane, Secretary of State George Shultz and Soviet Ambassador Anatoliy Dobrynin.

Earlier in the day, Mrs. Reagan visited a drug treatment center on the outskirts of Lausanne, ate McDonald's hamburgers on a boat ride on Lake Geneva with 25 flag-waving American schoolchildren and toured the picturesque village of St. Prex. As she walked along Grand Rue, some villagers leaned out their second-floor windows to wave in welcome. Tight security measures meant that most of the 100 or so residents in the old village center were not allowed out of their homes during Mrs. Reagan's stop.

At La Picholette, a drug treatment farm, Mrs. Reagan talked with six recovering addicts. One man, who had supported his drug habit as a prostitute, told her the program was "very hard but everyone who has the will can do it -- the willpower and courage."

Another resident asked her to "tell your husband that we would like lasting peace." She answered that "Everybody would, everybody would. I don't know anybody who likes anything but peace."

Hugging and kissing them, she told the residents she was proud of them. "If you go through this and out of it and back out, think of the number of people you can influence so their lives are not ruined."

Swiss patrol boats flanked the double-decker sightseeing boat during the 40-minute cruise between Lausanne and St. Prex. Icicles clung to the pier and to the land; pansies and chrysanthemums planted especially for her visit were frozen stiff.

After a folk band concert and a serenade by a group of Geneva singers, Gaston Schmidt, a farmer, called out, "Vive l'Amerique." He liked America, he told reporters, because "it saved Europe in two wars."