David Schwarz, the birthday boy himself, wore white tie and waistcoat, tails and Japanese plastic flip-flop sandals. Thus attired Saturday night, he greeted more than 100 guests, including his father, Robert Schwarz, the inventor of the Slinky toy, other architects, developers, artists and assorted people from Washington, New York, Fort Worth and Los Angeles gathered to sing "Happy Birthday, David." He was accompanied in the two-creature receiving line by his mostly basenji dog named Spike, who wore a stiff white formal collar and a black tie.

"I only wear shoes when it snows or freezes," said the bearded architect, who, when not celebrating his 35th birthday, is changing the face of much of Washington such as his Connecticut Avenue clock tower near Dupont Circle and the Griffin condominium in Foggy Bottom. "I considered wearing patent leather shoes for the occasion, but they're all plastic today."

His mother, Faith Lasser, who runs a medical institute, said he wears sandals because he has flat feet, inherited from his grandmother, who wore corrective shoes.

Schwarz's Japanese bathhouse footwear stood in contrast to the jazzy setting of his party. He chose not one of his own mid-'80s extravaganzas, but the fabled 1931 Art Deco ballroom of the Kennedy-Warren apartment building.

Mike Sutton of Schwarz's office headed an effort as architect-in-charge of the party (reminiscent of beaux arts balls of the turn of the century) to do modernistic decorations. Black screens with white translucent windows made a background on the stage behind Brooks Tegler's jazz band ("Good thing the marimba wasn't yellow," said Steve Jordan, the guitar player). Screens also covered the air conditioners until they had to be removed to cool down the hot time in the old town that night, despite the chilly outside temperature. Black balloons made a temporary sw,-1 sk,1 ceiling. "To rent the ballroom alone cost $600," said Eveline Carpenter Benning, administrator for Schwarz's 35-person office. Schwarz confessed that his birthday was a week earlier, but the ballroom wasn't available.

The black-and-white poster-sized invitation had read black-tie optional, and while a vast number of young men in their twenties and thirties chose to be formally costumed, there were variations. Jeffrey Schaeffer, a hairdresser (though not of Schwarz), wore a silvery battle jacket and a bracelet that might have suited Spike.

Sculptor Chris Gardner wore a woolly sweater. "I made the 10 table sculptures," he said, "because David told me he had this job, a sculpture for a seven-story building in Texas, but he said I had to do the birthday party centerpieces first." Kim Gardner, his wife and a Smithsonian designer, made the bases.

Hanne Merriman, Garfinckel's president, wore deep emerald green as background for an amazing brooch, appropriately in a '30s design. "I bought it with many more in Tokyo," she said.

Nothing was spared to give the illusion that the party was one vast late-late show: A young woman in a cigarette girl costume, complete with pillbox hat, sauntered through the ballroom, offering colored cigarettes. Although Glorious Foods has left Washington for New York, taking its birdnest settings and bird-sized bites with it, Mary Lee, formerly of the exquisite morsel caterers, moved over to Landsdowne caterers to oversee equally miniature morsels, including the ones Saturday night.

David Bonderman, for whom Schwarz has remodeled similar houses in Fort Worth and Washington, toasted the occasion by telling how he outbid Schwarz for a building and hired him to remodel it.

Faith Lasser paid tribute to her son, saying: "I know my son's successful, otherwise we all wouldn't be in this place tonight." And Schwarz said, "I figure I'll live to be 70, so this birthday is the beginning of my middle age."