"People who sit back and read the results of the yearling sales probably think we're a bunch of rich people with an easy racket," Marshall Jenney was saying this morning.

The Pennsylvania horse breeder did not display the confident composure of a rich man with an easy racket. Instead, he showed the understandable nervousness of a man who was about to see 90 percent of his annual income determined in one excruciating three-hour period.

Jenney's livelihood was resting principally on the spindly legs of a 1-year-old daughter of Tentam, who would go into the auction ring of the Saratoga Yearling Sales tonight. Hie equanimity was not helped by the knowledge that the price of his prize filly could swing $50,000 or $100,000, depending on the whims of a few buyers.

The yearling's value had already been affected amply by the whims of fate.

In 1976 Jenney went to a sale in France, spent $24,000 and brought a pregnant mare named Tananarive home to his Derry Meeting Farm in Cochranville, Pa. He made a quick profit when he sold the foal for $40,000.

Then he bred the mare to the stallion Tentam.

When Tananarive gave birth to a filly, Jenney was delighted. Breeders can spot potential defects when a thoroughbred first stands to nurse at his mother's side, but this baby looked physically perfect.

By the time the filly was weaned from her mother and put in a field with other yearlings, she was becoming a very special animal. The value of a horse's pedigree can fluctuate sharply: When a race horse wins a major stake (or better yet, a championship) his dam and her offspring immediately become more fashionable.

The daughter of Tananarive whom Jenney had previously sold was named Mrs. Penny and had been sent to Europe. There she was winning major stakes, and was voted the champion 2-year-old filly of England and Ireland. With his $24,000 investment, Jenney had done the breeding business' equivalent of striking gold.

Unlike bars of bullion, though, thoroughbreds cannot be stored for safekeeping. Jenney watched the filly develop as a nervous parent watches a child, and he remembered too vividly what happened when he was preparing to sell another outstanding prospect at Saratoga three years ago.

A groom was trimming a young filly's ears with a clipper when the animal got excited, flipped over, broke her back and died. Jenney's farm was plunged into the red for the year.

What happened this time was more subtle, less traumatic. But as Jenney watched the filly grow, he observed helplessly that her knees weren't developing quite right. They were what horsemen call "offset," a small biomechanical deficiency, but one that might affect her future soundness -- and her price.

"If her knees had been perfect, she would have brought $300,000 to $400,000," Jenney said. Those few centimeters of imperfection would, he thought, cut the price in half.

In the spring, Jenney started preparing the filly for the biggest night of her life. Yearlings don't have to do terribly much at Saratoga; they only have to stand and walk properly when prospective buyers inspect them, and present a good overall appearance. But all this takes conditioning. And with tens or hundreds of thousands of dollars at stake, all yearlings are taught very thoroughly to do these things.

"You have to make them stand with all four feet square so that their conformation shows the best advantage," Jenney said. "If their legs are crooked, you have to teach them to stand so not to accentuate the fault. You get them used to being rubbed by a groom. If a yearling is too nervous, you try to calm him down. If he's too skinny you try to fatten him up. We put on a yearling show at the farm to get them used to a little noise.

"This is one of the reasons I love the horse business. You have more control of your environment than you do in, say, the stock market. If some big fund dumps General Motors and you're holding the stock, there's nothing you can do. But if you've got a yearling with a problem, you can at least try to do something about it. Of course, you can wind up getting creamed, too."

When she arrived in Saratoga last week, the Tentam filly was resplendent -- as, indeed, most of the yearlings here are. She was attractively plump; her coat glistened. Outside her stall a sign called attention to her half-sister's achievements in Europe.

From morning till night Jenney greeted a steady stream of trainers and owners who would come to inspect his yearlings. All would utter niceties, but the breeder knew that is part of the etiquette of the business. He couldn't know whether trainers were whispering to owners, "Look at those knees! Forget this one."

Jenney chain-smoked through the evening until his filly was led into the auction ring at 11 p.m. The auctioneer extolled her virtues in the predictable fashion, and then opened the bidding at $50,000.

The bidding proceeded quickly to $120,000, where it stalled and sputtered. Aaron U. Jones, an Oregon lumber man bought the filly for $125,000.

Jenney's disappointment over the price had been somewhat offset earlier in the evening when Arab money and British money had bid another of his yearlings up to $330,000. It was, he knew, all part of the game.

"This is the nature of the business," he said. "You've got to be a gambler. And people think going to Las Vegas is exciting!"