On Friday night, 10 quarter horses will be loaded into the starting gate for the La Primera Derby. For a few moments, they will jump around skittishly. Then the bell will ring, the jockeys will whoop and start whipping furiously and the animals will start a 400-yard dash down a straightaway. Anybody who blinks may miss most of the action.

Not much drama can possibly unfold in the 19.9 seconds of the race, but it is charged with excitement nevertheless. The reason is money. This obscure event at little Los Alamitos Race Course is the richest horse race being run in America this week, richer than the stakes at Santa Anita, Aqueduct, Gulfstream Park or any other citadel of the thoroughbred sport. The second most lucrative race in the country this week is the Saturday night feature at Los Alamitos.

And the $275,000 pot for the La Primera Derby is small by quarter-horse standards. Los Alamitos offers three races with purses of $1 million or more, as much as any thoroughbred race in the world. The sport has come a long way since the days when a quarter-horse race consisted of two animals running down a dirt road to settle a bet.

The original members of the breed were working ranch horses, who had to be quick and maneuverable to do their jobs, such as cutting off a steer and separating him from the herd. Eventually, the best of them were reserved for informal competition.

Trainer Melvin Glaves remembers those days. "I grew up in Douglas, Kan.," he said. "We were farmers, but we always kept four or five match-race horses in the barn. Maybe 15 or 20 other people had horses, too. To make a match, you'd decide on the distance, and might decide that one horse would have to spot another one a length. Sometimes, we'd bet up to $500, and that was big money in those days. Then you'd take the horses and run 'em down a country road."

More formalized quarter-horse competition began at a few tracks in the 1940s, but, Glaves said, "It didn't amount to much. By the time you bought the (winner's circle) picture, it cost you money to win a race."

Quarter-horse racing was a small-time sport until Gene Hensley, the founder of a track in Ruidoso, N.M., transformed it with one big idea. Hensley recognized that the essence of the game had always been that owners put up their own money and raced their horses against each other. Why not try it on a grand scale?

He conceived a race for 2-year-olds, the All-American Futurity, which would require owners to pay a succession of nominating fees so they could keep their horses eligible. To encourage owners to make these payments, Hensley guaranteed that his race would be worth $50,000, double that of any existing quarter horse race.

Everybody thought he was crazy. They did, at least, until the first All-American Futurity, in 1959, offered a purse of $129,000. People in the quarter-horse industry suddenly saw the wave of the future.

Now an owner who wants to run a horse in the All-American Futurity has to make seven payments, totaling more than $3,000. So much money is raised in this way that the race offers a staggering $2.5 million purse, with $1 million to the winner. If the sport itself never stimulated much general public interest, this big money did and the economy of quarter-horse racing is booming.

But whether the stakes are $500 or $2.5 million, the object is speed. There are few subtleties that either jockeys or trainers need to worry about.

"Once you get a horse physically fit, your job is virtually over," said Wayne Lukas, who was the country's dominant quarter-horse trainer until he shifted to thoroughbreds a few years ago.

To get horses capable of delivering quick, out-of-the-gate speed, owners are relying on more than luck. The industry has a liberal definition of what a quarter horse is; interbreeding with thoroughbreds is permitted. And so owners have been trying to infuse their horses with the blood of the fastest thoroughbreds.

They have paid big money to breed to fleet thoroughbred stallions like Raise A Native and Barrera. One wealthy owner has been trying to breed a mare to Seattle Slew.

As a result, the whole breed is changing. At one time, the prototypical quarter horse was short and blocky. Now, most of the horses at Los Alamitos bear a great resemblance to thoroughbreds except for their short necks and unusually powerful shoulder muscles, which help generate their explosive fast start.

With millions of dollars at stake, quarter-horse owners had ample incentive to create the ultimate 440-yard running machine, and they have done it.