When Hal C.B. Clagett bought his first thoroughbred mare in 1947, he brought it home to his farm near Upper Marlboro, where it grazed among 500 acres of tobacco and corn. Today, Clagett's farm earns almost three-quarters of its revenue from breeding thoroughbreds, and old tobacco barns have been converted into horse stalls.

When horse racing enthusiasts Milton and Helen Polinger of Olney, in Montgomery County, decided to breed their own horses, they found costs so high that six years ago they opened a commercial farm and started breeding horses for others as well. Now they have three stallions and a farm that boards about 150 mares in the spring breeding season.

Clagett and the Polingers are in a business that has been part of the Maryland countryside since George Washington came to Upper Marlboro and lost money in races held by the Maryland Jockey Club. Today that business is better than ever, thanks to state-sponsored cash incentives coupled with soaring prices and profits.

Racing business experts estimate the money spent in all areas of the racing business in Maryland has quadrupled in the last 10 years, with an annual cash flow of nearly $500 million today. Breeders have gained more than anyone else.

Statewide, about 25 percent of the money Maryland farms invest in livestock is spent on race horses. While raising standardbred horses -- used for harness racing at tracks, such as Prince George's Rosecroft Raceway -- is flourishing, by far the largest breeding business in the state is thoroughbreds. Most of those horses will race at Pimlico in Baltimore, the Bowie Race Course in Prince George's, the Laurel Race Course in Anne Arundel County and the smaller track at Timonium in Baltimore County.

The number of horse farms, even in suburban Montgomery and Prince George's, is rising: The number grew from 53 to 62 farms in Montgomery over the past 10 years and from 20 to 26 in Prince George's. Baltimore County tops the list, with 150 registered thoroughbred farms.

As horse farms have increased, so have the winnings of Maryland-bred horses, experts said. Last year, the state's throughbreds won 141 stakes races, surpassing the 1979 record of 118 wins, and raked in almost $5 million in purses in the United States alone.

This year, the 6-year-old filly Jameela, which now races in California, became the biggest Maryland-bred money winner ever, earning $1 million since 1979. The Kentucky-bred gelding John Henry is the world's top money winner, with a total of $3.4 million, and is still racing.

"The industry has just taken off," said Snowden Carter, manager of the Maryland Horsebreeders' Association. "People have realized it's a good investment, with a lot of romance. But it's a hazardous industry. You can get a good score and make an inordinate profit, but you can lose big, too."

For someone like Carter, who recalls that $2,500 stud fees seemed expensive 20 years ago, a horse like Northern Dancer, which is stabled at Windfield Farm in Chesapeake City in Cecil County, is almost frightening.

Northern Dancer, considered America's greatest living stud and the sire of hundreds of race-track winners, earns a $250,000 stud fee. Members of the syndicate that owns the horse recently refused a $40 million offer to buy the 21-year-old stallion. The stallion was not bred in Maryland, Carter said, but "everyone knows where Northern Dancer stands; it means a lot to the state."

Northern Dancer, of course, is not a typical horse. "The drop between Northern dancer and the number two stallion is a million miles," Carter said.

Sir Ivor Again, a Maryland-bred horse and the senior of three stallions at Pollinger Farms Corp., is the highest-priced stud in Montgomery County at $7,000. But a stallion can sire up to about 60 foals a year, and the money adds up.

The long, yellow stables of the Polinger farm will house about 150 thoroughbreds during the spring breeding season, most of them visiting mares. The stud fee is only collected if a foal is born, so farm workers take great pains to keep the mares healthy and to keep production up. "Each one of these animals is a piece of real estate," said Don Ryan, son of Helen Polinger and manager of the Polinger farm.

Clagett is a different sort of breeder, whose horse stalls made from aged tobacco barns contrast with the modern stables of the Polinger Farm Corp. Clagett learned his trade by experience and experiment since he bought his first mare, Glimpse, in 1947, shortly after he graduated from law school, began his practice in Upper Marlboro and took over the family farm.

Now he has 20 broodmares, which he takes to other farms, like the Polingers', to be bred in the spring. Each spring, he has at least five young horses to sell, for an average of $25,000 each. He estimated the thoroughbreds on his farm are usually worth $750,000 to $1 million.

"A tremendous transition has taken place on the farm, reflective of the economic changes in the county," Clagett said. Many of the fields have been divided into paddocks, and barbed wire has been replaced with board fences so the thoroughbreds will not be injured.

He said the farm never would have survived if he had kept it as a tobacco and grain farm. Clagett's ancestors, buried in the farm graveyard where Clagett himself will one day lie, depended on slave labor and later on sharecroppers.

On a tobacco and grain farm today, "you just can't meet the minimum wage and survive," he said. But as a horse breeder, he said, "happily, I won't have to break up my farm and can provide a semblance of a decent living."

Horse breeding started getting even bigger in Maryland in the 1960s after the Maryland legislature created the Maryland Fund Program, which allocated one cent of every dollar bet at Maryland race tracks as cash prizes awarded to Maryland-bred racers and champion stallions and as cash incentives for top Maryland breeders.

In the past 10 years, the amount of money distributed from the fund each year has grown from $1.5 million to $3.8 million. The Polinger horses, for instance, earned $14,749 in bonuses last year. John A. Manfuso, a Seat Pleasant lawyer who has a thoroughbred farm in Carroll County, received more than $12,000. Hal Clagett's brother Fendall M. Clagett, who has a horse farm in Anne Arundel County, received more than $24,000 in bonuses. "It's a real incentive," Carter said.

Unfortunately, he noted, it has been imitated and refined by other states. New York State, for instance, has a program that gives out more than $8 million a year. While Maryland's horse business has grown, the state has dropped from being the fourth largest breeding state (after Kentucky, California and Florida) to the sixth (after Louisiana and Washington state).

But this is not a fair measurement, said Robert Lawrence, a professor at the University of Maryland's College of Agriculture who has studied the state's horse breeding business. If you consider quality, he said, Maryland would rank in the top four. The state still ranks second after Kentucky in the number of horses bred per acre, he noted.

Lawrence said breeders have gained more than any other area of the horse-racing business in the past 10 years. Most breeders sell their horses as yearlings, he noted; in 1974, 4,100 yearlings were auctioned across the country, at an average of $9,200 each. Last year, about 8,000 yearlings were sold, for an average of $35,000.

In Maryland, he estimated, the annual cash flow in the racing and breeding business jumped from $165 million in 1970 to about $450 million in 1980, with about 36 percent of it going to breeders, owners and trainers. This year, the state expects to collect almost $15 million in racing taxes, which goes into the state's general funds. Another $4 million is fed back to the race courses themselves.

Carter of the horse breeders' association said he is confident thoroughbred breeding in Maryland is about to have another spurt of growth. The association this year became partners with the Maryland Sales Agency, a new horse auctioning business, and Carter said sales at Timonium, where the association holds several auctions a year, already have picked up. With high horse sales in Maryland, he noted, breeding horses for sale will become an even better business.

Furthermore, he said, the state legislature this year voted to give $100,000 to the Maryland Fund Program if association members will donate the same amount. (Association members plan a fair later this year to raise the money.) This isn't much money, he said, but it is an important indication the state is willing to support horse breeding.

Closer to home, Clagett predicted horse breeding in Prince George's will continue to grow. "Prince George's County has always been one of the traditional places where breeding and racing has flourished," he said. "We have the land, the climate and the facilities that makes this one of the real areas for future growth."

With an abandoned race track in Upper Marlboro recently rehabilitated into an equestrian center, Clagett said, it could develop into an important training ground for racehorses headed for tracks in Bowie, Laurel, Pimlico and Timonium and could encourage breeding in the area.

"It's at an embryo stage now," he said. "It will grow tremendously. It's the only direction a farm can grow. It's old and antique in many ways, but it's ready to burgeon."