PARTYING in costume is a way for artists to be creative and sociable at the same time.
When Ellen Frank, whose studio is in Springs, completes her plans for a medieval themeevent later this year, it will augment a tradition that has produced amusement, sparkle and memories, along with a chance to try out alternative identities for a brief while.

Eastern Long Island can trace its history of artists using costumes back to Thomas Moran (1837-1926), who collected a range of costumes, as Ms. Frank does today.

A series of 1980's costume balls by the Parrish Art Museum and by the Artists Alliance of East Hampton (originally named the Jimmy Ernst Artists Alliance) have left rich recollections of creative costuming.

The cartoonist Charles Addams was the judge for the Parrish Museum's 1984 edition of its Beaux-Arts Ball, to which guests were asked to dress as a favorite work of art. ''Just about every artist on Eastern Long Island participated,'' said Anke Jackson, a museum official.

The concept for the event, modeled after the Beaux-Arts Ball in Paris, was developed by a member of the museum's Region Council, Gayle Willson, in the hope of creating a warmer relationship between the institution and local artists.

''Artists really got into it here,'' Ms. Willson recalled, describing one guest dressed as a Magritte painting, complete with a black hat and green apples.

''They did some mammoth things,'' she added. ''There were multi-artist costumes, like a Frederick Remington sculpture group that included cowboy, Indian and horse; an interpretation of Picasso's 'Guernica' mural; and a white-painted group recreating a George Segal sculpture. That cluster of artists even danced together all night.''

She has special memories of Rocco Liccardi's creations, including a daring tableau based on Pieter Bruegel's painting ''The Massacre of the Innocents.'' And Mr. Liccardi speaks fondly of his prize-winning costumes based on a Jackson Pollock drip painting and on a Georgia O'Keeffe depiction of a cow skull.

''People change when they go into costume, and projects like this trigger the imagination,'' he said. ''It's fascinating to see what people come up with, and it's fun to see what you can do in terms of making a costume.''

Another ambitious award-winning creation was based on Leonardo da Vinci's mural ''The Last Supper.''

''We rehearsed our poses,'' recalled Ian Hornak, an East Hampton artist who took one of the 13 roles. ''At the proper moment for judging, we all struck our positions in front of our huge, cloth-covered table, painted with grapes, bread and goblets.''
He also has special memories of a costumed tableau that suggested a Persian miniature. ''A dozen of us surrounded the central figure, who was carried in an ornate sedan chair,'' he said. ''That creation won the top prize, too.''

Both the costumes and the decorations made memories at the series of Red, Black and White Balls that the Artists Alliance presented for a decade, first at a firehouse in Springs and later at Guild Hall, the television studio at LTV in Wainscott and the Parrish Museum. Themes included old movies, a night at the opera, vampires, the Roaring 20's, the circus, outer space and science fiction.

''To carry out the space exploration idea, we suspended a foam board rocket ship with red paper streamers from the ceiling air-conditioning vent, so the stream of air simulated the fiery vibrations of a rocket blast,'' said Brian Leaver, an Amagansett artist who conceived and executed the decorations once the annual theme was determined. ''We also had tiny white lights attached to black plastic sheets covering the walls.''

Surrealism was Mr. Leaver's favorite costume party theme. After placing black photography paper over the walls, he applied a tumbling white stream-of-consciousness line over the entire surface.
''These were terrific projects, for a good cause, and doing something for a group of artists was gratifying,'' Mr. Leaver said. ''Since we usually could get access to a space only two days before the event, and there was really no money available, it was a challenge, but one I enjoyed. The freedom the project allowed was a welcome challenge, too, and the physicality involved in drawing rapidly on such a large scale was wonderful.''

Although there has not been an Artists Alliance ball since 1995, the organization hopes to revive the project. ''Artists love to express themselves like this,'' said the group's president, Diana Plitt. ''It appeals to their theatrical nature, and they can impress their peers.''

Amy Zerner, a Springs artist, said that being in costume transforms people. ''It inspires them to get in touch with other energies,'' she said, ''and it changes the chemistry of a room when people use their imaginations like that.''