Le Tout Paris had a rendezvous with Le Tout Washington last night and both participants found the experience formidable, magnifique.

The occasion was a soiree at the French Embassy held to fete the 475 actors, singers, dancers and orchestra musicians who had participated in the three-ring opening night of the "Paris: The Romantic Epoch" festival at the Kennedy Center. The members of the Comedie Francaise, the Orchestre de Paris and its chorus and the Stuttgart Ballet mingled in a polyglot group of more than 1,000 guest who enjoyed a late buffet dinner in the embassy and in two tents erected outside to catch the overflow.

"I'm thinking of buying the tents," said Ambassador Francois de Laboulaye. "I'm beginning to like them."

"If he gives a party like this everyday," added Mme. de Laboulaye, "he'll have to change wives."

The French artists seemed very pleased with the warm reception they received opening night in all three of the Kennedy Center's large auditoriums.

Pierre Dux, Director of the Comedie Francaise, was beaming after the performance of Victor Hugo's "Ruy Blas." "Everything went very well," he said, "including the simultaneous translation. The accoustics in the Eisenhower Theater are marvelous."

Francois Beaulieu, who plays the title role in "Ruy Blas," looked almost as dashing in his black tie at the embassy as he had earlier in a Spanish renaissance costume complete with sword. "The response of the American audience to this French play was very warm," he said, "but it was also very subtle. We had the feeling that we had an audience that understood and appreciated what we were doing. This is my first trip to the United States and I am combining several pleasures-doing work that I love in a country that I love for a public that I love."

The evening began less smoothly for the Stuttgart Ballet, whose opening was delayed 25 minutes by final preparation of the 103 complicated lighting cues which had to be worked out for the Opera House in only two days. But this performance, too, ended in triumph. Later Martin Feinstein, artistic director of the Kennedy Center, recalled that he had sent out the German dancers with the good-luck wish that is traditional for them, although meaningless to most others: "Toi, toi, toi." The French actors and musicians in the Kennedy Center's other two houses were sent on stage, he said, with their traditional good luck wish: "Merde."

Among the Parisians at the party was the Countess Isabelle de Lasteyrie du Saillant, sister of French president Valery Giscard d'Estaing, and an official in the Ministry of Culture. Dressed in a red chiffon gown by Guy Laroche, the countess said that she was in the United States working at her specialty, music, not representing her brother. The French, she told another guest, are "still a very romantic country. There is now a romantic club of young people; a trend is starting, and nobody knows where it will lead." The French Minister of Culture and Media, Jean Philippe Lecat, agreed with the countess that romanticism is prevalent "in the younger generation-alas, not in my own."

"One of our large newspapers recently conducted a poll of young people," he said, "and discovered that they consider love more important in their lives than employment, schools, or anything else."

An accent of youth was given to the party by the presence of 180 members of the chorus. "It is still a very new chorus, established by Daniel Barenboim in 1975, and most of its members are quite young," said Jean Lecat. One of the young members, in the middle of a conversation in French, was complimented on the accent-free diction with which the group had sung "The Star Spangled Banner." "I am British," she said, still speaking French. "We have 18 nationalities in this chorus," and then the conversation shifted into English since it was the native language of both speakers.

Also speaking French frequently during the evening was Martin Feinstein, whose accent was pure Berlitz. Asked for his reaction to the gala and hyperactive evening which he had been working for years to prepare. Feinstein commented, "Je suis unpeu fatique."

Patrick Hayes, who is with Feinstein the chief importer of musical talent into Washington, paused to pay him a graceful compliment that incorporates part of the name of an English chamber orchestra: "They're beginning to call you Saint Martin in the field," said Hayes.

"I am very far from sainthood," replied Feinstein, for once almost at a loss for words.