The rate of spontaneous abortions is reported to be as high as 75 percent on some thoroughbred farms. And higher-than-normal rates of stillbirths are also being seen this spring among foals bred last year.

That has prompted a few panicked mare owners to move their horses out of state and has scientists at the University of Kentucky's Gluck Equine Research Center scrambling for answers.

''This is devastating. It is potentially the biggest crisis we've ever faced in the industry,'' said Gus Koch, a longtime manager at Claiborne Farm, one the industry's leading farms.

Koch normally expects to lose one fetus by this time in the breeding season, which began Feb. 14. Tests over the weekend found 11 mares had aborted, he said. Abortion, rather than miscarriage, is the term used by The Jockey Club, which registers North American thoroughbreds.

Kentucky is the world leader in thoroughbred breeding, with more than 20,000 mares bred in Kentucky last year, far more than any other state. No one knows exactly how much income the industry generates for the state, but in 1997 the Barents Group estimated that the entire horse industry in Kentucky had a direct economic impact of $1.2 billion.

Farms began noticing the high abortion rate last week -- about one week after Central Kentucky veterinarians began to see an unusually high rate of ''red bag'' births of foals that were conceived last year. In such births the placenta separates and comes out before the foal, said Dr. David Byars, hospital director at Hagyard-Davidson-McGee Associates, an equine veterinary practice in Lexington.

Those foals often suffocate and are stillborn. Or they are sent to the hospital with breathing problems, severe dehydration and low counts of white blood cells, which fight infection.

''We've had a really high mortality rate with those foals that have come into the hospital, around 50 percent,'' Byars said. ''Then Derby week we began hearing complaints about high percentages of fetal losses.''

Farms sometimes use ultrasound tests to determine the sex of a fetus at 60 days. What farms have been finding are a large percentage of fetuses with no heartbeat, or empty wombs in mares that had once been pregnant. The mares show no signs of illness, according to Koch and a handful of other farm managers.

Dr. David Powell, a veterinary epidemiologist at the Gluck center, said the high abortion rate is widespread throughout Kentucky. More than 20 researchers and veterinarians are working full time to try to find out why it is happening.

At this point, researchers believe the deaths of both fetuses and foals may have the same cause.

The leading theory is that Kentucky's unusual spring weather this year -- warm and dry early on, followed by a hard freeze -- set the stage for a fungus or toxin to develop in the field grasses eaten by horses.

Based on that, Powell and others are advising farms to mow in order to reduce horses' exposure to bluegrass and other grasses.

''The advice we're giving is to keep the horses in well-mown pastures,'' he said. However, he added, ''we haven't eliminated the possibility that it is an infection of some kind.''

Investigators have ruled out little, but they can say the cause is not equine herpes, which also causes abortion. Powell also said horses are immune to foot-andmouth disease, which has afflicted other livestock in Europe.

''There is no connection between what we're seeing here and what's been happening in the United Kingdom,'' he said.

Also, because the foal and fetal deaths occurred suddenly in many locations, that would suggest it's not a contagious disease, Powell said. That would have hit one farm, then another in domino fashion.

Pasture management specialists are analyzing grass samples, while some of the aborted fetuses are being examined for any infection. Other scientists are looking at the increase this year in caterpillars, which may carry disease, Powell said.

The Gluck center has sent questionnaires to all farms to try to determine the extent of losses and see if any patterns emerge. Veterinary researchers will discuss the deaths at a news conference scheduled for this afternoon at the Gluck center.

Whatever the cause, the financial losses to Kentucky's multibillion-dollar horse industry could be heavy.

''It will be millions. How many millions we don't know,'' said Ric Waldman, a thoroughbred consultant for Overbrook Farm near Lexington.

Overbrook has been luckier than most, with an abortion rate of about 10 percent. But that's still higher than normal, Waldman said.

THE LOSSES will be felt first by stallion owners, who will forgo stud fees in many cases. Stud fees typically are based on what's called a ''live foal guarantee,'' meaning that the mare owner doesn't pay anything until a foal is born.

Overbrook is home to the country's leading stallion, Storm Cat, with a $400,000 stud fee. In some cases, his stud fee is based on a live foal guarantee, Waldman said.

When a foal is not guaranteed, mare owners often insure themselves against the loss of a foal, said Alex Rankin, a Louisville insurance agent who specializes in the horse industry.

It's too early to tell how many claims will be made, he said.

But he said the abortion epidemic ''has made it difficult, if not impossible, to write foal insurance.''

Rankin breeds horses in Oldham County and said that area has seen similar problems.

The financial losses also extend to mare owners, who may lose the use of their mare for a year -- in addition to having no foal to race or sell.

The damage is especially bad because it involves two years of foal crops -- 2001 and 2002, said David Switzer, executive director of the Kentucky Thoroughbred Association.

Keeneland Association, the Lexington race track and auction house, oversaw the sale of $750 million worth of horses last year. Sales surely will suffer with fewer Kentuckybred foals available, said Geoffrey Russell, assistant sales director.

''You can't sell if you don't have the product,'' he said. ''Right now, we're more concerned with the farms. They're the ones that are really going to suffer.''

Waldman said the crisis could bankrupt some smaller, struggling farms.

''THE LARGE commercial operations should be OK, but if you're a marginal farm, it doesn't take much to put you over the edge,'' he said.

Powell, of the Gluck center, said he has received some reports that breeding farms in other states also had reported problems. But none could be confirmed, he said.

Debbie Pabst, owner of Blue Ribbon Farm in Washington state, said her state doesn't appear to be having the same problems.

She sent three mares to a stud farm near Paris, Ky., this year and all of them are still in foal, but she worries about what is causing the problems and whether it will affect her horses.

Jerry McMahon, president of Barrett's, a thoroughbred auction house in Pomona, Calif., said he hasn't heard of similar problems in the West.

''We've had no reports of this incident in California. I presume we would have heard from some of our breeders if anything like this was going on here,'' he said.

McMahon said problems with foaling horses in Kentucky ''would send a ripple effect through the entire industry'' if the abortions and stillbirths persist -- especially if the problems were to expand to other places on the East Coast and to other regions.

''Hopefully it won't come to that,'' McMahon said.

Staff writer Joseph Gerth contributed to this story.