It is, by any measure, an extraordinary bazaar, a collection of limitless values of the most luxurious and extravagant items celebrating the vanity of men and women - over the past 400 years.

It is The Metropolitan Museum of Art's current fashion exhibition, called Vanity Fair, 500 or so item selected from 30,000 of the museum's own treasure trove (The Costume Institute) by special consultant Diana Vreeland, the lady with the shrewdest eye for such things.

It's Vreeland's sixth effort for the museum, and the dirst in which she has dipped solely into the museum's own resources. Last year's exhibition, an exceptional display of The Glory of Russian Costume, drew nearly 850,000 visitors.

It is not just the range of items, from tiny shoes from China to the Duke of Windsor's culottes, from Worth ballgowns to Norma Kamali parachute-cloth outfits, that makes this "bazaar" such a treasure. Unlike the shabbey shape of the items in most bazaars, everything here is in top condition.

To show them otherwise would be like showing damaged paintings. "We want you to imagine you were back in the time of these clothes, not in a thrift shop," says Stella Blum, The Costume Institute's curator. "These were the great clothes of their day and we wouldn't be telling a perfect story with sweat marks or tears."

The clothes are the memorabilia of social change - recalling the tastes, styles, needs and sometimes foolishness of past eras. One can almost trace the changing role of women, for example, from the increasing sturdiness of their shoes. What did it matter if heels once were set under the arch if it made a woman's foot look smaller - and she had no place to go anyway? Likewise corsets.

Queen Alexandra, wife of Czar Nicholes, sits sidesaddle on a black stallion in her Worth-designed riding outfit, and five ladies in riding habits of other perods surround her.

Another mannequin wears the black net and jet-sequinned Arnold Scaasi gown designed for sculptor Louise Novelson - and the necklace of hard-ware and fiddle parts and the 10-gallon hat she chose to wear with it.

The elaborateness of men's dressing gowns worn in the 18th Century are a surprise; so are the turbans men wore at home when they took off their wigs.

Then there were times when vanity went a bit berserk. A brocaded court gown from England in 1750 has such huge side paniers that the wearer could only enter a room sideways. And there are huge ornamental combs (some three feet wide) worn by ladies in Argentina in the 1860s. And broad bicorn hats for men of the Napoleonic era and hatboxes in matching shapes.

The Costume Institute's collection comes mostly from donations, boosted by a small acquisition fund. "We don't look at where things come from or who wore them - only if it is the best of a period," says Blum, who admits she's getting fussier and fussier about the condition of thing she accepts.

Many of these items arrived in well-worn torn, altered and even abused condition, probably unrecognizable except to a curator. That's where senior restorer Elizabeth Lawrence takes over, studying, watching and even "taking," she says, to the garments and accessories to get a sense of what they were originally.

"Fabric has a memory," she finds, and a garments gives clues to its original construction and "balance" as well as the shape it takes from the warmth of the body of the wearer. "Sometimes you can even begin to see that fat pads of the woman from the spill over the top of the corset."

Repairs, even when done with a made-to-match fabric, are sewn so the professional eye can tell the clothes have been repaired, and so they can be returned to the condition in which they came to the museum. A patch in a virtually identical fabric used to hide a sweat stain, for example, is topstiched over the damaged area. "The point is not to fake it," says curator Blum firmly, "but to make it worth showing."

Lawrence, who once ran the work-rooms for Farquharson and Wheelock, a custom dress house on 57th Street, relies on 50 volunteers, most of whom have worked some time in New York's garment district, plus a number of students.

Threads, sequins, notions and the like come from old mending bags, but when the proper material isn't available, they create it. Fabric is tinted for a perfect "match." Ingenious students have copied Schiaparelli buttons and a Vionnet buckle in dental materials and identically paired up a missing sleeve with a handpianted fabric.

Pharmaceutical talcum powder successfully lifts some stains and hides others. Skillful display lighting helps to disguise some irreparable damage.

Lawrence's responsibility doesn't end until the garment is actually on display. For the 50 or so 19th-century lingerie gowns, endowed with ribbons and lace. Lawrence and volunteers hand-washed and ironed them all and stitched each ruffle in nylon to make them stand up. Starch would have rotted the fabric, she says.

It is that room full of exquisite lingerie that is expected to have the biggest influence on American fashion designers who are already on a linen and lace kick.

There are five study rooms where designers and researchers may tap the resources of the museum and study specific styles. At least one American designer is said to gain many ideas on cut and style from this source, though credit is never given.

Storerooms are temperature and humidity-controlled, with garments hung on smooth plastic hangers or laid flat in drawers on acid-free paper. Cabinets are Fomica since wood contains acid. Lights are low, and the air is filtered. "Pollution is disastrous to fabric," says Blum.

Vreeland chose the items strictly for their "prettiness" and displayed them in term of style rather than the way they were actually worn. Lace dresses are shown without underpinnings and velvet gown by Madame Grey designed in 1977 is shown with Schiaparelli gloves, vintage 1939.

"The public isn't concerned about ponderous accuracy," Vreeland told a reported at the opening last month. "They want spectacle . . . the elusive spirit."

The show will close in September.