"Nice party," said Martin Feinstein, with the slightest of grins, to J. Carter Brown, director of the National Gallery, as the two of them stood among champagne-sipping guests on the Garden Terrace of the Gallery's East Building, overlooking dusk and the Capitol.

"Be it ever so humble," Brown responded, breaking into the broadest of grins.

It was hardly a humble affair held for the 80 or so guests dressed in black tie or chiffon, some of whom had flown directly from France for the opening of one of the Kennedy Center's greatest extravaganzas-a festival of arts celebrating 19th-century Paris, and bringing the Comedie Francaise, the Stuttgart Ballet, and the Orchestre de Paris, for starters.

To supplement the theme, the National Gallery opened an exhibit last night which features four French Romantic paintings, three of them from the Louvre and one from the local Phillips Collection.

And to add to the recreation of the creative spirit that ran rampant through 19-century Paris, pianist Byron Janis was invited to play Chopin-just a few feet away from the Delacroix portrait of Chopin-after dinner.

"You'd never have this in New York," said the Kennedy Center's Roger Stevens, referring to the festivities and the cultural meetings of France and Washington. By evening's end, after Janis finished playing, Carter Brown fairly danced out of the Gallery recital humming Chopin as he went. "You have to be happy after something like that," said Brown.

The French guests, who included the Countess de Lasteyrie du Saillant, the sister of French President Valery Giscard d'Estaing, and Jean-Phillippe Lecat, French minister of Culture, raved over the National Gallery.

"There's nothing to compare with this in France," said the Countess du Saillant, who works for the Ministry of Culture.

And that included the Centre de Georges Pompidou, a multi-arts center built in the last decade, which is often compared to the National Gallery. However, some of the French guests went so far as to say the Pompidou center was "awful" compared to the National Gallery.

"Congress seems very fond of the Gallery," Stevenson was heard to remark affectionately to French Ambassador Francoise de Laboulaye.

Among the guests were Mrs. Arthur Hartman, wife of the U.S. ambassador to France, and her daughter Sarah; Nicole Salinger, wife of Pierre, who lives in Paris; Grand Duke Jean and Grand Duchess Charlotte of Luxembourg; Patricia Berberg, in whose Paris apartment Janis, Chopin, Liszt and Brahms have played (the last three before her time); and the mayor of the village of Sache, Paul Metardier and his wife. Sache is where artist Alexander Calder maintained a studio for 25 years.

On the local side, Frank Ikard, Washington attorney and member of the Kennedy Center board and his wife Jayne were there. So was Mrs. Jouett Shouse, donor of Wolf Trap Farm Park, who said that conductor Sarah Caldwell and opera singer Shirley Verrett had just been added to the lineup of artists who will appear at the June 1 Wolf Trap Gala. J. Carter Brown's wife Pam was there, wearing a periwinkle blue chiffon dress, by New York designer Julio.

Mayor Marion Barry and his wife Effi had to cancel their attendance at the reception and dinner. "It's a good crowd to cancel out of," said one person. "I don't know how good his French is."

La nouvelle cuisine, although highly touted for its exquisite light touch of the sauces and the cooking, was a little heavy on the derriere. The seven course meal, estimated to take two hours to serve, ran behind schedule, with about 25 minutes between courses.

When it was all over, Lecat called it "a very romantic menu-the most romantic menu I have ever experienced."

The meal, the creation of Yannick Cam, chef at Le Pavillon Restaurant here, was a masterpiece of seafood, fowl, and roast lamb. White chocolate mousse, chocolate truffles with Armagnac and almond cookies were the dessert course. There were three wines-all French.

The same meal in a restaurant, one expert estimates, would cost around $80 a person, or $6,400 for the last night's guests.

Pianist Janis did not mind sitting down to the lengthy dinner before sitting down to play Chopin in the gallery of the French Romantic paintings.

"It isn't the usual thing," Janis said with a smile. "But we're doing this in the spirit of the salon feeling-so it's an easy, relaxed thing."

Janis said at the reception he might play from one of the four unpublished manuscripts of Chopin compositions that he has found.

Janis found two earlier unknown version of the G Flat Waltz an the Grand Valse Brillante in Paris in 1967. Later, he found two other earlier variations, unpublished, of the same two waltzes in a library at Yale in 1973.

The Delacroix "Portrait of Chopin," considered a masterpiece, is from the Louvre, as is "Dante and Virgil in Hell," also painted by Delacroix. The third painting from the Louvre is Gustave Courbet's "Portrait of Berlioz." The Phillips contribution to the exhibit is a Delacroix portrait of the frenzied, virtuoso violinist Niccolo Paganini. Before the reception Carter Brown took Lecat, Stevenson, Ambassador and Mrs. de Laboulaye, and the Countess du Saillant on a tour of the exhibition and parts of the Gallery. Part of the tour he conducted in flawless French.

Martin Feinstein, who is responsible for bringing the extravaganza, "Paris: The Romantic Epoch," to the Kennedy Center, did not look like a man who had spent the last weekend making sure that about 400 performers and crew arrived here from Europe and made it to hotels and rehearsals on time.

"There's a certain amount of tension when you have something so mammoth in all your theaters at once," he said, the picture of friendly calm. Of all the various festivals that he has put together-such as LaScala and the Bolshoi-he said "this is probably the most important-because it is all the arts from one period together." CAPTION: Picture, Ambassador and Mrs. de Laboulaye with Mrs. J. Carter Brown; by Joe Heiberger-The Washington Post