SHORTLY AFTER 6 o'clock Friday evening, art dealer H.H. Leonards started greeting her guests. Barefoot and sporting a cream-colored housedress, the first of a half-dozen outfits she would wear over the next two sleepless days, she was trapped in the narrow entrance hall of her towering Victorian manse, behind an antique gaming table atop a 120-year-old Kurdish runner and between two massive paintings leaning on either wall.

She brushed long brown strands out of ever-widening eyes. "Oh, HELLO," she said breathlessly. "I haven't combed my hair."

So began "The Third Annual HOT Mid-Summer Forty-Eight (48!!!) Hour H-Athon," a manic celebration of commerce and insomnia on a quiet stretch of O Street NW, to which hundreds flocked over the weekend to browse, gawk, doze, babble--and buy.

"This has never been a profitable venture," Leonards said as it ended. Still effusive, though somewhat paler and by then swathed in a black kimono, she was marking her 42nd hour of wakefulness. "I do it for the pure joy of it and the psychology behind it--meeting new people and having them see the house."

Leonards, who shares the striking four-story building, brimming with wood panel and filigree, with lawyer/husband Stuart Pape, had put up for sale nearly all its contents, most of which she keeps on consignment: thousands of paintings, etchings and sculptures, plus hundreds of antiques and pieces of bric-a-brac, including the gaming table ($850), carpet ($3,000) and the Chippendale chair ($16,000 for the set of 16) on which she held court. Almost everything but her three boxers, who kept vigil over the proceedings. Dangling discounts, she encouraged bargain hunters to show up at odd hours (between 2. a.m. and 6 a.m.) and wear odd costumes.

Neighbor Patricia Gorman, for the sake of a 30 percent rake-off on six paintings and lithographs, arrived at 2 on Saturday morning, a white-haired Samoyed dog in tow. Joyce Bouvier, a legal secretary, came around the same time as an Arabian temptress, plunked down $150 for an Elie Abrahamie lithograph titled "The Bar," and departed in time to catch a bus for the Poconos.

Late Saturday night, as white-jacketed butlers carried drink trays through the throng--a contingent of which, in Panama hats, had collapsed on an Empire sofa--a chap from Alexandria arrived garbed as a guru while Bob Hammack, a Department of Energy auditor, showed up as a geisha and his friend Rob Moon, a Navy lieutenant commander, turned out in a 1927-vintage full-dress ensign's uniform.

"It's authentic; I inherited it from my Uncle Burt," Moon said. "We came for a lithograph by Papart. He's a French cubist, you know."

And Roger Rihm and wife Ashley Christina Sprague--all in red, with Rihm sporting a rubber hose around his neck, Sprague wearing a mask with grill-work and flames--showed up as a fireman and fire engine, respectively. They mingled and made siren noises. "We wanted to bring a Dalmation, but we couldn't find one anywhere," Sprague said. They bought, at a 30 percent discount, a lithograph by Frenchman Michel Delacroix.

Leonards and her husband bought the striking house three years ago, and she promptly turned it into a warehouse for her art and antique trade. Very little--from the ancient Japanese temple garden ornament standing in the back parlor to the complete set of Napoleon III dishware in the dining room--stays around for more than a month.

"It was unsettling for a while," Stuart Pape said. He was keeping a low profile for the H-Athon, spending much of his time sunning himself by the pool out back and reading Gabriel Marquez's "One Hundred Years of Solitude." "I still remember the first time I came into the living room to sit in my favorite chair and ended up falling on the floor. I guess the only thing that's permanent is the stuff in our bedroom."

"No, it's not permanent," H.H. chimed in. "If somebody offers me a fair price, I'll take it."

"NO WAY," Pape shouted, slapping his hands together, his face a mask of well-tanned horror.