Tom Gentry lay on the floor of a stall, cradling the head of a 1-day-old colt, while his veterinarian held aloft a bottle of fluid that trickled through a tube into the animal's veins.

The colt's mother had died giving birth the night before; the vet had to cut open her side and pull out the foal. They had been repeating the intravenous feeding throughout the morning in an effort to save him.

Gentry looked up from the straw. "Just remember this," the breeder told a visitor, "when you see somebody sell a million-dollar yearling in the summer sale and you start to think what a piece of cake this business is."

The spectacular prices commanded by well-bred horses and the glamor of the yearling sales tend to obscure the risks and difficulties inherent in the thoroughbred-breeding industry. It is a business that deals with a frighteningly fragile product. It is a business dependent on the whims of nature: the chance that a stallion's sperm will fertilize a mare's egg and produce a million-dollar baby.

America's breeding industry has achieved success and preeminence partly because it applied such science and skill in dealing with these risks, in attempting to get a maximum number of mares pregnant with a minimum number of stud services. Such serious matters can't be left to nature. The breeding season -- Feb. 15 to June 15 -- doesn't even coincide with the season nature intended. Breeders found that pituitary glands are photosensitive, so they keep horses under lights during the winter. "Under the Light," a breeder explained, "the pituitary wakes up and says, 'Hey boys! It's time to smell the roses!'"

Breeders continually monitor the fertility of their stallions and mares, leaving as little as possible to chance. At Gentry's farm, a vet makes regular rounds, checking the mares by putting on a plastic glove and reaching inside them.

"Sara Lee is F3L strong, very fragile," he says, indicating to Gentry the size and condition of the follicle that contains the eggs. Gentry gets on the phone and tries to schedule her mating to Stop the Music -- which he had arranged months earlier -- at the optimal time for conception.

All this attention has paid off measurably. Not long ago stallions would average 2 1/2 covers for every mare to whom they were booked. Now the average is close to 1 1/2, which enables stallions to service a greater number of mares. With stud fees as high as $ 225,000, those extra matings mean many extra dollars.

But the technical superiority of American breeders is only a small part of the reason for their success. "We are breeding the soundest and toughest and best horses in the world," proclaimed Spendthrift Farm's president, Brownell Combs, and the large number of foreign buyers attending the Keeneland Selected Yearling Sale in Lexington, Ky., today verifies that his opinion is not mere chauvinism.

In a sense, the modern-day bloodstock boom had its antecedents in the late 1940s, when American breeders -- armed with strong U.S. dollars -- bought and imported many European stallions who infused vigor in American pedigrees. When Kentucky breeder Bull Hancock put together a syndicate that bought Nasrullah for $340,000 and imported him from Ireland, that transaction alone helped change the nature of American racing history. Nasrullah was this country's leading sire five different years; his son Bold Ruler would be the leading sire seven consecutive years.

But America didn't achieve success simply by raiding Europe after the war. Racing historian Kent Hollingsworth has another explanation for its preeminence: "In English races the horses gallop a mile and a quarter, then turn loose the last quarter-mile. A horse runs three or four times, then wins the Derby, and they retire him. But our 35,000 foals a year are subjected to the toughest racing regimen anywhere. They run 10 times a year or more on hard dirt tracks -- and they run hard all the way. The survivors are a damned tough breed."

Even if our selection process was producing tougher and faster horses, the rest of the world didn't acknowledge it. Europeans looked on American breeding with ill-concealed contempt; they thought we were speedcrazy, producing horses without the stamina to win classic races on the continment. They didn't want our horses. In 1969, only two of the 60 most expensive yearlings sold at auction in this country were bought by a foreigner.

But by the early 1970s, Europeans had to make a painful reappraisal of the American thoroughbred. In 1968, 1970, 1971 and 1972, American-bred horses won the Epsom Derby. They were starting to achieve similar success all over the world. Ted Bassett, president of the Keeneland Association, can boast, "In the last decade, horses sold at the Keeneland sale have won every major classic in Europe."

Foreign buyers began to converge on U.S. horse sales and, with the dollar weak, they could afford to outbid Americans for their own horses. Two years ago Gentry sold a record $1.6 million yearling to Japanese investors. Last year that mark was erased when a Greek shipping magnate outbid an Irish group for a $1.7 million colt. Almost half the money spent at the Keeneland yearling sale last summer came from abroad. Once the object of derision, the American thoroughbred had become a commodity that the whole world coveted.