"I felt a little like Cinderella. I had the dress for one night and it disappeared," said Nancy Reagan, almost voiceless from laryngitis, at a luncheon yesterday at the Museum of American History.

"It was hard to give up, very hard," she added, repeating the story later at the official presentation of the handsome, beaded, one-shoulder inaugural ball gown to the Smithsonian Institution's First Ladies exhibit.

Like Cinderella's gown, this was no ordinary evening dress. Six women worked six weeks just to apply the beadwork and embroidery, said designer James Galanos after the ceremony. Mrs. Reagan asked for a one-shoulder style, a personal favorite, but left the rest of the design to him. He gave her the dress, which he said was "beyond the $10,000 value that was quoted," with the idea that she eventually would donate it to the First Ladies exhibit.

"The hall is the first place where the carpet wears out," S. Dillon Ripley, secretary of the Smithsonian, told 250 invited guests and a great crowd of others in the "We the People" hall.

Mrs. Reagan understood why the exhibit is so popular. "Women are always curious about what other women wear," she said.

Walking to the podium with Ripley, Mrs. Reagan barely glanced at the plastic mannequin that had just been revealed from behind a folding screen. But prompted by photographers, Mrs. Reagan, wearing a plaid suit by the same designer, posed as still as her likeness.

It was the elaborate craftsmanship as well as the design in the gown that led Mrs. Reagan during lunch with Ripley six weeks ago to suggest a First Ladies Fellowship for study of American costume at the Smithsonian. "We have a wonderful tradition of clothing design and craftsmanship in America. It is not always appreciated, and my hope is that the fellowship will fill this need," Mrs. Reagan said. The fellowship, which is for predoctoral and postdoctoral scholars, is expected to be funded by private interests.

After the short ceremony, the Nancy Reagan likeness was rolled into the costume hall and placed in a period setting of the White House Red Room, in the same case with the First Ladies gowns from Jackie Kennedy on.

"Well, it doesn't quite look like me," Mrs. Reagan whispered after looking at the gown in the case. "But I have very special memories of wearing the dress that night. It is very sentimental."

The mannequins in the exhibit were never planned to have facial likenesses of the First Ladies. "Cordelia, the daughter of King Lear, is the model for all the faces," said Ripley, referring to the bust by Pierce Connelly that is used as a mask for the faces.

The Reagan mannequin, built to Mrs. Reagan's measurements in the Smithsonian's laboratory, has the hairstyle she usually wears, not the pulled-back style she wore to the inaugural balls. The more recognizable short, curly style was Mrs. Reagan's choice and was sculpted on the mannequin by Julius Bengtsson, her favorite California hairstylist. "It was wrong at first and it wasn't easy to change," said Bengtsson yesterday. "I got the chisel out. But I'm a hairdresser, not a sculptor."

Yet even the severest critic, the designer, liked the view of the gown under glass. "It looks very effective, it has a nice feeling," said Galanos before racing off for a plane back to California.