Though not always able to find exactly what they were looking for, the 800 guests at last night's black tie preview for the 31st annual Washington Antiques Show at the Shoreham Hotel agreed the quality of pieces was superb.

Room-size rugs were running between $5,000 and $40,000 and for a mere $265,000 you could own a 1770 Chippendale block front desk.

"There isn't any junk and there are no bargains," remarked Mrs. K. Martin Worthy, an avid antiques collector. "Many pieces are close to museum quality," she said.

"You would have to travel great distances to see the range of dealers in this show," said Knight Kiplinger, a veteran of 20 shows. He and his wife Ann were looking for mirrors and other furnishings for their new home but ended up instead buying a camphor-lined Chinese chest. And William Joyce was disappointed that he couldn't find andirons with Hessian soldiers on them. "There must be 400 different andirons here," his wife Mary said, "but not the Hessian andirons."

Although Honorary Chairman Barbara Bush was unable to attend, Clement Conger, curator of the White House, the State Department Fine Arts Collection and Blair House, could be seen carefully browsing through several of the 44 booths. "I've found lots of things that piqued my interest," he said, including "Chinese porcelains at Elinor Gordon's" and an $8,000 Philadelphia bracket clock. "And I saw a Heriz rug that might do in the entrance hall of the State Department." But for the moment, Conger didn't commit himself to buying.

DAR President Sarah M. King chatted with guests as she was looking for a highboy for the Constitution Hall reception room.

Show cochairman Mary Roddy was particularly pleased with the exhibit loaned this year by the Winterthur Museum in Delaware, which tied into the show's theme "Collections of the Decorative Arts in America."

In addition to bringing fine antiques to Washington, the show also benefits the Thrift Shop Charities, which include the Children's Hospital National Medical Center, the Child Health Center Board of Children's Hospital, Columbia Hospital for Women, the Hospital for Sick Children and St. John's Child Development Center.

The show will be open to the public today through Sunday at the Shoreham.