In an oversize sandbox (66 by 197 feet, to be exact), the trials begin. A blond, freckle-faced woman trots an adroitly stepping dappled gray across the ring. Her radiant smile hides her concentration on the foot-high letters placed along the edge of the ring that mark exactly where they are to perform.
Thirty feet away, in the warmup arena, fine tuning is under way: ''He's moving like a drunken sailor,'' the British trainer reprimands a rider on a chestnut mare. ''Now pick him up, pick him up; gently, gently, keep him well contained. Sit right into him. Now send him forward.'' And the choreography begins.
The horse rocks into its gaits with a cadenced, playful spring. As though becoming airborne, its stride extends and elevates. Pivoting on its haunches, the horse sidesteps across the warmup ring and comes to statue-like halt. The trainer nods approval. ''Now he's ready,'' he says.
What they're so intently engaged in is dressage. The Mercedes-Benz of horse sports, this French term (pronounced like ''massage'') means ''training.'' In the midsummer heat here, 58 riders were vying for a coveted place on the dressage team of the United States Equestrian Team that will compete at the World Championships in Switzerland next week (Aug. 25-29).
Once as exotic here as Perrier used to be, this equine art is making itself known to the American riding public. Membership in the United States Dressage Federation (USDF), which promotes the sport through sponsoring and organizing competitions, has grown 500 percent over the past decade, from 2,400 to 12,500.
''Thirty years ago, dressage was known by only a few. Now it ranks among the fastest-growing equestrian sports,'' says Lowell Boomer, executive secretary of the USDF.
It doesn't offer the high stakes of racing, the glitter of the show world, or the crashing thrills of rodeo. Rather, the drama of dressage lies in appreciating the precision and exacting technique of the performers. As Ellin Dixon, a leading young rider, explains, ''When you put it together right, it's an equine ballet.''
To the uninitiated it is baffling: horses circling in an arena cut off from the lush countryside. But within the swirl of hay, fly spray, and sweat one can sense the intensity of the sport.
Except for the horses' rhythmic snorts and the muffled sound of hooves hitting sand, the grounds are quiet. A group of maybe 200 spectators grouped in the shade of pine trees watch in silence, or whisper to one another in French, German, or in British-accented English, as each horse and rider perform their solo test.
Five judges score the quality of each movement. The horse and rider, like figure skaters, must perform the pattern of twists, turns, sudden changes of gait and speed, with absolute precision and no apparent effort.
But dressage is more than technique.
''Dressage horses dance,'' says Kay Meredith, president of the USDF and a top competitor. Having just finished the test, she is reddened and perspiring. When her score is announced, it is but one-tenth of a point short of winning. An innocent toss of the head or a moment's hesitation may have cost her the victory.
Originally a form of self-defense, dressage was first used on the battlefields of Europe 400 years ago. For a knight to survive in battle, his horse had to respond immediately to every command. Whether sidestepping an oncoming aggressor, pivoting instantly, or charging forward, his life depended on the horse's agility and quickness. Over the centuries, dressage has remained a formal method of riding especially popular in Europe.
Joy Cowan, a competitor, expresses her affection for the sport with a kaleidoscope of comparisons. ''It's what it must be like tuning a fine race car . . . creating beautiful music . . . or balancing colors in a painting that says exactly what you want.''
Encyclopedic books have been written on dressage that analyze the muscular, structural, psychological, and emotional responses of the horse. Ironically, the complexity of dressage lies in its simplicity, devotees say. The hardest part of training is convincing the horse he can perform a natural movement with a rider on his back, says Joy Cowan. ''Often his instinctive reaction is to tighten his muscles. My aim is to produce the most balanced, supple gait with the horse obediently responding. In other words, the most pleasant horse to ride.''
For some, dressage is an esoteric philosophy in which the horse becomes an extension of the rider.
What keeps Joy going even if weather's bad or she's not feeling up to par is the anticipation of that moment of complete harmony. ''When it works,'' she says , ''it's a perfectly synchronized feeling.'' With this goal she has learned to appreciate self-discipline and find enjoyment in her daily work, she says.
Karl Mikolka, a former rider of the famous white Lippizaner stallions at the Spanish Riding School in Vienna, insists that dressage is the only way of fully developing the horse's athletic ability.
Yet even among riders, dressage is often unappreciated. ''I hated dressage. I used to think it was all a bag of circus tricks,'' says Sandy Pflueger, a rider selected for the US team who learned to ride herding cattle on her father's farm in Hawaii. With the help of her coach, she says, ''I realized that it was all very logical. It just takes a long time to put into practice.''
Although an Olympic sport, dressage remains a novelty to an American public accustomed to hard-hitting sports action. And unless royalty happens to be present, dressage events are rarely covered by the news media. On this estate outside New York, however, the secrets of dressage were unveiled.
The art of training is diplomacy, says Mr. Boomer. ''Horses aren't like golf or tennis balls. Everything in horsemanship is based on reward and punishment, and the subtlety with which it is carried out makes you either a genius or a failure.''
To reach the highest levels of international competition may take five years of training. The horse, rider, and trainer become an inseparable threesome, ironing out faults each day on their way through about eight levels of competition to the Grand Prix test, the summit of dressage.
Sustaining motivation is the hardest part, riders say. ''But the few moments when it works is worth all the agony,'' says Linda Oliver, a leading competitor.
''It's the hardest career I can imagine, but I can't think of a better hobby, '' says Priscilla Hergesheimer, president of the New England Dressage Association. ''What I find so exciting about it is that there are always new horizons, every day you're learning something new about yourself and the horse.''
Back at the arena in another corner, a black horse with a coat polished to an enamel sheen abruptly stops, reverses, and lurches forward against the rider's spurs. Incensed by the roughness, her German trainer corrects in thick Bavarian tones, ''No, that's no good. Try again. Be more positive. Don't force him when he's not ready.''
European trainers have capitalized on the American enthusiasm for the sport. It's a compatible relationship in which Europeans provide the expertise and Americans a Yankee determination to win.
''I'm probably much more obnoxious than I used to be,'' says Ellin Dixon humorously. The 25-year-old college senior manages to combine international dressage competition and the ''real world.'' Through working with her 1,000 -pounds-plus counterpart, she explains, ''I've become a much more confident person.'' Intent on becoming a history teacher, she adds with a laugh, ''Maybe by 90 I'll have my PhD.''
To accommodate both interests she's up at 5 a.m., studies for an hour, rides five to six horses, and then in the afternoon and evening returns to her academics. Yet she is the exception.
Sandy Pflueger, for example, follows a much stricter regimen. ''I realized I could become good in many things but never excellent in any one thing unless I devoted myself solely to that,'' she says.
As a result she focuses every day on her equine pursuits. Waking at 6, she doesn't stop training until 7 at night. While her horses get one day off a week, on that seventh day she often has to ride other horses she has missed earlier.
With so much work required for relatively little tangible reward, what keeps people so committed to its goals? Robert Dover, one of the few male competitors in America, laughingly admits that it's because he's a ''lunatic like everybody else in the sport.'' He says, ''I love horses, and I want to get the most enjoyment out of them I can.''
When dressage is performed correctly, the horse shares in the enjoyment, riders claim. ''The most challenging part is to keep the horse mentally and physically happy, and this requires complete harmony of body and mind,'' says Karl Mikolka. He adds with possibly a little bias that dressage is the most humane way of training a horse.
Not only do dressage horses have a longer exhibition career, but also riders have competed at the Olympics in their 70s.