"A man mounted on his horse is twice the man he is on the ground," said Diana Vreeland, fashion expert extraordinaire and consultant to the Costume Institute of the Metropolitan Museum of Art. "Such a man holds the reins of power and progress in his hands, for the horse has been the basis of the mobility of culture."

"Man and the Horse" and costumes for both are the focus of Vreeland's 13th annual Costume Institute exhibition, which was previewed at a gala benefit dinner dance tonight and opens to the public on Dec. 18.

For years Vreeland dreamed of doing this show. "I was brought up with horses -- they were part of my life," she said. Born and raised in Paris, she could hear the clicking of horses' hooves in the Bois de Boulogne from her mother's apartment.

"I never had a doll, I had horses," she wrote in her whimsical autobiography "DV," referring to the toy horses she kept lined up in tiny stalls on one side of her room as a child. She also observed, "Horses smell much better than people, I can tell you that."

She loved to ride. "Oh yes, it was a period when we all rode, and I was mad about horses," she said. "I haven't ridden in a long, long time, but I rode stride, as I lived in Europe at that time. My boots were from Peale of London , and the precision and workmanship of everything one wore riding was wonderful.

"There is about horsemanship and clothes a kind of perfection of workmanship . . . anything to do with outfitting a horse is perfection," she added firmly.

A few of the 850 glitterati who paid $750 a ticket for dinner and a crowded glimpse of the exhibition this year wore formal riding gear, including pink jackets. More than 2,000 others paid $125 to come after the dinner, in part to see the show but perhaps more to catch a glimpse of the guests, among them Nancy and Henry Kissinger, Evangeline Bruce, William Paley, Cheryl Tiegs, Louise Nevelson, Paloma Picasso, designers Oscar de la Renta, Bill Blass and Perry Ellis, and other members of the Council of Fashion Designers, which cohosted the evening. And, of course, Ralph Lauren, who gave $350,000 to underwrite the evening.

Many of the dinner guests ride horses and lent things for the exhibit or were consulted on the protocol of man and his horse. "Both you and your horse must be well turned out," said C.Z. Guest, who recalled that for a horse show in Washington her horse had 30 braids on his neck that were so perfect that they looked as if they were done by hairdresser Alexandre of Paris.

Pamela Brown agreed that most serious riders demand everything be perfect, down to the polished boots. "He knows what sticklers they are," she said smiling at her husband, National Gallery of Art director J. Carter Brown. "He's married to one."

New York Sen. Alfonse D'Amato, who attended the show with cosmetic magnate Estee Lauder, had less fond horse memories. "I went riding twice and promised the good Lord that if he spared me I'd never go riding again."

Following dinner in a fancifully recreated Scottish castle, complete with 12 deer heads wrapped in tartan mufflers, guests danced in the Temple of Dendur to the music of Peter Duchin and in the Charles Engelhard Court to music by Pe De Boi, a Brazilian band.

Vreeland's first thought was to call her show "Horse and the Man," concentrating only on the way man has dressed the horse, confided her assistant, Stephen Jamail. "In the course of discussions with the museum, the emphasis got turned around," he said.

Vreeland wasn't surprised to learn that her exhibit coincided with that of George Stubbs' paintings of horses and riders now at the Tate Gallery in London and later to travel to New Haven. In fact, she could acquire only one painting by the British artist for "Man and the Horse" -- from her chum Bill Blass.

"I do believe in rhythm, don't you? You have to," she said. "Rhythm in the way that I think now that people want to get outdoors and feel the air and know what is really happening in the world." According to Jamail, the fact that an American, Joe Fargis, won two Olympic gold medals in equestrian events this summer confirmed Vreeland's sense that this was the right moment for the show.

Man has not been the same since he got up on his high horse more than 4,000 years ago, in Mesopotamia. It changed not only his perspective on the world but accelerated his ability to see more of it. "When you are on a great horse, you have the best seat you will ever have," Winston Churchill is quoted as saying in the catalogue accompanying the exhibit.

But it affected as well man's posture and his apparel both on the horse and off. The first time a man wore something resembling trousers, it was to protect himself from chafing on a horse, says Jean Druesedow, associate curator-in-charge of the Costume Institute.

"It is an interesting fact that almost every article of sporting attire worn by men and women today throughout the world, and indeed almost all male attire, is derived by direct descent from the hunting clothes of England," wrote the late James Laver, noted fashion historian and keeper of prints and drawings at the Victoria and Albert Museum in London.

In the preface to "Clothes and the Horse," Sydney Barney's guide to correct dress for all riding occasions, which was used as a handbook for "Man and the Horse," Laver added, "One might say it was the horse who invented sports clothes; at least he compelled their invention, which amounts to very much the same thing."

Apparel is, of course, the main emphasis in this exhibition, which features 150 examples of men's and women's equestrian costumes. But as in all of Vreeland's efforts, it goes beyond clothes to include even more accouterments related to the horse -- saddles, bridles, crops, bits, stirrups, horse blankets and carriages, as well as paintings and sculpture. The opening scenario looks like a tack room, displaying some extraordinary saddles, bridles and a saddle cover used by Queen Elizabeth, one of three pieces borrowed from Buckingham Palace.

At least two-thirds of the habits for riding, hunting, racing and polo playing are for men. A 14th-century knight in shining armor is mounted on a horse in armor, and there's a liveried royal coach attendant from France circa 1825, clothes for the hunt and a colorful range of racing silks.

Every rider wears boots, a problem not only for dressers of the mannequins, but for one young volunteer, Kirk Adair, who spent three months polishing them. "Mrs. Vreeland loves leather and boots, and they had to be perfect," said Jamail.

Women first wore jackets identical to men's for horseback riding in the 17th century, but paired them with skirts. The influence of the men's jacket is still apparent in the earliest women's riding attire in the exhibition: five jackets from the 18th century, with cuffs and button treatments and pockets like those of the men's jackets placed nearby.

"The equestrian world is not a theatrical world. One dresses down to perfection," Vreeland wrote in the catalogue introduction. And it is that focus on correctness that is the limitation of this show. Because the rules of just what is appropriate for both rider and horse are so clearly prescribed, the originality and charm -- and even the personality of Vreeland, so clear in her previous exhibitions -- is somewhat curbed.

Not entirely, of course. Vreeland has lined a sleigh in a sable coat someone donated to the Costume Institute. And one mannequin modeling an early woman's costume is shown riding a carousel horse.

"In some ways it is more difficult to mount a show with such precise requirements," said Druesedow. Tremendous care was taken "to do everything by the book, to make sure every tie was correctly tied." Experts on everything from horsemanship to carriage driving were brought in to make sure those ties were properly tied and whips properly placed.

Druesedow was impressed with the precise tailoring of all of the garments. "Everything is very neat and tidy. Everything is tailored to within an inch of its life," she said. In the 19th century, "riding skirts were tailored to a woman sitting on her saddle in a tailor's shop so that each knee would fit perfectly in a pouch in the skirt that kept it in place. Sidesaddle breeches worn under skirts were fitted individually as well, because each leg was in a different position and looked as right while on the horse as when one got off."

She is a bit less sure about the use of the word "pink" to describe the formal red coats of the hunt. "I wonder if the best explanation isn't that it was derived from British military apparel. You fought in scarlet, hunted in red and danced in pink. Perhaps you wore your well-worn and faded coats -- your dowdies -- to dance in," she theorized.

But Ambassador Charles Whitehouse, who was wearing a scarlet tail coat from the Orange County Hunt in The Plains gave the final word on why such coats were called pink: "Pink was the name of the tailor."

Not only riders, but grooms and even passengers in coaches followed rules of appropriate color and cut in their attire, also on exhibit. Doors to the Costume Institute were removed so that the 15 coaches could be brought in.

There are no bareback riding clothes or jeans in the exhibit, and other than the Olympic uniform no contemporary garb. Many of the clothes, however, would be appropriate today, including Coco Chanel's Harris tweed hacking jacket and brown twill jodhpurs, which she is said to have copied from the attire of a friend's groom.

There is no sign, therefore, of Ralph Lauren's man-on-a-horse logo, the symbol of his Polo designs since 1967. Dressed in jeans, Lauren toured the exhibit early this morning. "A lot of it looks very familiar," he said. The appeal, he explained, is that it represents a quality of life and sportsmanship that everyone wants to associate with. "It is natural and rugged and not really fashion. Like jogging shoes and Army surplus. It is the good life," Lauren said.