In the early morning hours when Barney O'Donnell's greyhounds get their exercise at their kennel in Hialeah, thoroughbreds are training at the famed race track just a few miles away. The contrast between the two scenes is extreme.

Horses are led through the well manicured stable area and sent to the track, where they gallop past a backdrop of palm trees, a setting that epitomizes the beauty of the sport.

The greyhounds are loaded into the back of a pickup truck and driven to the "sprint track," a long, narrow strip of dirt surrounded by a wire fence. There they run back and forth, or at least they are supposed to. If they are not in the mood, one of O'Donnell's men will drive the truck outside the fence, taking advantage of dogs' universal tendency to chase after moving vehicles. Glamourous it is not.

Besides the esthetics of their training procedures, there is another sharp contrast between the horses and the dogs. Thoroughbreds cost more than $30 a day in training expenses, and nine out of 10 of them will lose money for their owners. But the greyhounds are almost guaranteed moneymakers. Even before O'Donnell has had an opportunity to judge the ability of an individual dog, he can expect that there is an 80 percent chance that the animal will earn a profit for him.

O'Donnell's dogs get their first athletic experience at the age of seven months, when they are permitted to chase (and sometimes catch) live rabbits on a farm. This may give them the incentive to pursue mechanical rabbits in the future, but they probably would pursue them, anyway.

"Greyhounds have a natural instinct to run and chase," O'Donnell said. "You can tie a rag on the back of the truck and they'll chase it." Only about one dog in 100 will display utter lack of interest in competition. His life expectancy is short.

While a trainer does not need to teach a dog to run, he cannot teach a dog to run well. When a 10-month-old greyhound starts his preparation for competition and runs alone around a small track, he may display a tendency to run on the outside or on te rail. He probably will keep that tendency all his life. A human cannot alter a dog's nature. "All we can do is keep them clean, Healthy and well-fed," said Chuck Lambert, O'Donnell's trainer at the Hollywood Greyhound Track.

Just a few years ago, O'Donnell said, only about half of his dogs were good enough for race-track competition. Most of the others had to be destroyed, becuse there was no place to run them profitably and because greyhounds are not friendly little pooches who can be adopted as house pets.

But now that greyhound racing has grown so much, with tracks in such unlikely places as Eutaw, Ala., and Pownal, Vt., even untalented dogs can find a home. O'Donnell finds it advantageous to run a far-flung operation, competing at six or more tracks at a time, so he can keep his better dogs at the big-league tracks like Hollywood and dispatch the lesser ones to tracks like Green Mountain.

At all of the tracks, racing is conducted in a way that practically insures the kennel a profit. Kennel owners sign a contract with the track management, guaranteeing to provide them a certain number of dogs throughout the meeting. Outsiders may not enter the races, except for stakes. If a man has a fast greyhound in his backyard and wants to race him at Hollywood, he must lease the dog to one of the contracted kennel owners there. The kennel owner will then pay all of the dog's training expenses and keep 65 percent of his earnings.

This system insures that every kennel will get a piece of the pie, that owning dogs will be profitable even though the purse money is small in relation to that of the horses. At Hollywood, only 28 kennels are permitted to operate. There is a long waiting list of dog owners hoping to join the chosen few, and for good reason.

"Of the 28 kennels here," said the track's general manager, Perrine Palmer, "I would guess that the top 10 in earnings make a handsome profit, the next 10 make a reasonable profit and the bottom eight barely break even."

Of course, it wasn't always this way, and O'Donnell can remember the days when everyone in the dog business was struggling. But that was before new dog tracks began opening frequently, before major stakes races were offering $125,000 purses, before the sport had become a tremendous growth industry.

"Dogs have come so far in the last five years, it's hard to believe," O'Donnell said. "It's like seeing a flower bloom right in front of your eyes."