It is unlike any benefit you would ever see in proper Washington. But here in the capital of the thoroughbred breeding industry, the hostess not only gets away with it, she gets ever more famous for it. It is the annual benefit for the Fayette County Heart Fund and the Bluegrass Boys Ranch, otherwise known far and wide in this part of the world, as The Madden Party.

Phyllis George, the sportscaster who married Kentucky Governor John Y. Brown, has called it "simply the best party in America." It is Anita Madden's annual Derby eve revelry held under a huge red tent on the polo grounds of Hamburg Place, the 2,000-acre horsebreeding farm she and Preston Madden call home. Each year the event gets bigger and each year Anita Madden has to come up with something spectacular to make the party memorable. One year, six people streaked through the crowd.

The theme this year was "The Diamond as Big as the Ritz," after the F. Scott Fitzgerald fantasy about a boy who goes home with a schoolmate to the family's diamond mountain. The family guards its secret mountain with antiaircraft guns, but in the end, aviators discover it and bomb it.

Near the entrance of the tent, a young woman wearing a brown stone on her decolletage perched on a Rolls-Royce. The stone was the 111.59-carat Earth Star, the 31st largest diamond in the world, sent to the party by the De Beers mining company.

No expense was spared. The rhinestone-covered canopy over the dance floor was patterned after the car that took the schoolboy to the mountain. About 1,500 guests ate a sit-down dinner (served whenever they wanted to eat) at 150 tables decorated with white spider mums set in vases of crystal chips. There was champagne and wine on the tables, mixed drinks at open bars. The ladies' room was a triumph in ingenuity. After all, one can't equip a tent with indoor plumbing for 1,500 people, just for one party a year. On the other hand, one can't expect Kentucky's horsy set to use Don's Johns. Anita Madden's solution to this delicate question was to erect a large lounge just outside the front of the tent, swathe the walls in red and gold fabric, and create five large separate wallpapered cubicles, with portable commodes and plenty of lighting. Hoses brought water onto the field for the sinks.

Anita Madden made a spectacular entrance around 10 p.m. wearing a red sequined gown and a black cape covered with mirrors, with a back that stood up several feet. She is tall, thin, beautiful and talks with the accent of her native Ashland, Ky., which she describes as the "hillbilly part of the state. The joke in Ashland is when hillbillies die and go to heaven, they come to Lexington." She met Preston Madden, grandson of the legendary Kentucky horse breeder, at the University of Kentucky and went to heaven. They have a son, Patrick, a freshman at Stanford. Anita Madden's parties are not the only events that have made her famous. She doesn't suffer outrage gladly, unless it's her own. She recently scolded the state racing commission, of which she is a member, for allowing Marfa, the Derby favorite, to keep the winnings from a race he shouldn't have entered. Last week, she picketed a TV station that was running a series on a friend of hers that she thought was unfair.

Not long after her entrance, the entertainment began. The music changed and soon the go-go dancer on the elevated stage became a topless go-go dancer. There was a light show, and then huge model airplanes swooped across the top of the tent, complete with pyrotechnics. On another elevated stage, a male dancer appeared in a tuxedo and a female dancer appeared in a flapper outfit. Before long, they were dancing more and wearing less and the audience was applauding and whistling. As the dance concluded rather memorably, Anita Madden, looked up from her conversation, widened her eyes and murmured decorously: "I didn't plan this!"

But then, as she reflected in the quiet aftermath of the 109th Derby, "in the spirit of Derby week, you can do a little bit more than you can during the rest of the year."

You couldn't, of course in Washington, where it's important to take everything and everyone seriously, and where ultra-elaborate parties these days would prompt raised eyebrows. But it's nice to know that in the heartlands, at least, there is still somebody who is not afraid to be a little outrageous. Anita Madden gets away with it by doing things very, very well.