-- Spanish explorers brought the horse to the New World, and the first herds cantered onto the Montana high plains around 1730. Almost immediately, it can be assumed, somebody here got on a horse that didn't like it and promptly fell off.

This interaction between human and horse is replayed again and again during three days every May in one of America's most exhilarating -- and remote -- parties: the annual Miles City Bucking Horse Sale, which concluded here Sunday. By day it is a lot like a rodeo, but it isn't a rodeo. At night it is a street celebration billed as the "cowboy Mardi Gras," but that doesn't get it right either.

This is a raw, beer-soaked anachronism closer in mood and action to the annual bullfighting festival in Pamplona, Spain, than to anything else, though you won't hear many people in eastern Montana use the word aficionado.

Miles City, about 150 miles east of Billings, with a population of 8,400, is one of two incorporated towns in Custer County. The other one, Ismay, has 20 residents.

The sale almost doubles the population here. It takes place across from the stockyard, at the southwest edge of town, on the windswept threshold of a stark landscape once known as "the Great American Desert." It's not called that on maps anymore, but few people here would argue with the description. For as far as one can see, the landscape is all prairie grass and sage, seared and brown, rising and falling over steep hillsides against a horizon broken by sandstone buttes standing under high clouds that often threaten rain but rarely bring it.

"This is the real West," said Bill Broadbent, an institutional broker with Lehman Bros. in New York, who was here for his 14th consecutive Bucking Horse Sale. "This is one of the greatest unspoiled places left in the Lower 48 states. And it's also one of the best parties."

At the time, it was either late Saturday night or early Sunday morning, and Broadbent was in front of the packed Montana Bar. The action on Main Street was just beginning in earnest. There were a few couples twirling through their two-steps, with boot heels digging into the pavement. But they were lost in a larger, oceanic swelling of people.

The mostly young, decidedly unsteady crowd moved from one jammed bar to the next. Lean men in tall hats eyed groups of women in low-slung jeans and loose, shimmery tops weaving paths among the cowboys, a lurid mix of laughter and cigarette smoke and bawdy invitation rising into the chilly spring air to a thumping beat from the bands lining the sidewalks.

"This is fabulous!" Stephanie Seitz hollered over a song from a group called 37 Clark, which was playing on a flatbed trailer parked between the Montana and the Hole-in-the-Wall cafe. Seitz, who just graduated from Texas Christian University, is from McAllen, Tex. She came to the Bucking Horse Sale with a friend from college.

"There's a lot of stuff going on here that I haven't been exposed to before. The bars are fun, but I think the actual sale is really the best part," she said.

That is true. When it isn't about drinking, which is for a few reverent moments before the first Bloody Mary is poured each morning, the Bucking Horse Sale is about horses and cowboys. The sale goes back to the late 1800s, when nearby Fort Keogh was a remount station for the Army. The rodeo has long since replaced the cavalry as a market for unbroken horses, and this is one of the largest bucking horse auctions in the country. Organizers expected the sale of upward of 250 horses.

Miles City is famous for its roughstock -- "green" horses that either have not been ridden or cannot be ridden. The horses are "bucked out," which is to say ridden for a few seconds, by local cowboys, young amateurs with dreams of making it in professional rodeo someday. Buyers standing in the ring bid on the animals immediately after each ride, so that the event itself seesaws between spectacular, intensely brief rides, and the rattle and roll of an auction. In between come horse races and bull riding. Spectators, most drinking a beer with one hand while dangling the balance of the six-pack from an opened loop of plastic, mill and cheer and gasp appreciatively.

Riding a green horse is something nobody is really eager to do. The horses are unpredictable, and the risk of injury is high. A few come out and buck magnificently, pitching forward and back, heads down, forelegs planting stiffly and hind legs kicking high in the air. That's the horse you want, said Spike Buffington, an affable Miles City saddle-bronc rider who is a rising star. Like all the cowboys here, he looks the part, chiseled and bandylegged.

"Riding a good horse is the only way to pull a paycheck," Buffington, 20, said.

Russ Moses, a rodeo operator from Jackson Hole, Wyo., said he looks for a big horse, preferably a mare he can breed. Geldings are much less valuable. Nobody likes studs because, well, they just have a bad attitude.

"I look at the horses in the corral," Moses said. "They don't know each other so you can tell a lot from how they act. I want to see who's boss. That's the one I want."

Buffington's first ride on Saturday afternoon was predictably unpredictable. It was aboard a 4-year-old, never-ridden gelding owned by Bill Klukken. When a saddle was put on him in the chute, the horse blew up -- kicking and twisting, squatting with its head down and then back over, until it had smashed out several two-by-six wooden slats and managed to turn half onto its back. "He's very spirited," an expectant Klukken said.

Buffington finally got on, leaned back, hat low over his eyes, and nodded. The chute opened and the horse slowly turned -- and then galloped gently across the ring.

Since the sale is not a sanctioned rodeo event, anybody can ride. One bareback rider who signed up on a whim was Alex Ireland, 16, from Gig Harbor, Wash., who happened to be passing through Miles City on a vacation with her mother. Before Saturday, she had ridden only a few practice rounds on easier horses back home. Waiting her turn behind the ring, Ireland -- a willowy 5 feet 4 inches tall -- looked a lot smaller and younger than the other cowboys. But she seemed calm.

"Then I'm fooling you," she said, adding, "I don't care what kind of horse I get. Hopefully, it'll be a nice one."

It wasn't.

Horse number 552 in the sale was a no-name gray gelding that stood passively in the chute as Ireland lowered herself into position. But when the gate swung open, the horse whirled violently to the left and in the same movement gave a huge, ugly buck toward the clear blue sky. Ireland was thrown high and clear, twisting once and falling in a dainty heap onto the dirt.

Later, sitting on a fence gate, Ireland said she was not sure whether she would go again that day.

"It was really fun," she said, beaming. "I opened my eyes, and I was just spinning through space. But we'll see about riding later. My head hurts a little, and I feel kind of dizzy."

There was a lot of that going around.

Cowboys hang on the fence to stay out of the way as they watch a bucking horse attack its rider Saturday at the Miles City Bucking Horse Sale.