The brown show horse was getting his mane and tail braided early yesterday morning when he suddenly dashed for freedom through an open barn gate. His groomer and owner chased him but could not keep pace as the horse galloped across a grassy field, onto a still-dark Prince George's highway and careened into an oncoming car, killing himself and the driver.

Frances Imwalle, 39, a resident of Lothian in Anne Arundel County, was driving north on Route 4 when the horse bolted in front of her 1996 Ford Taurus.

"It was too late to make any speed corrections," said Maryland State Police Cpl. Jeffrey Stevenson. "She had no alternative but to strike the horse."

The accident happened minutes before 5 a.m. in Upper Marlboro, the county's rural-suburban county seat, just blocks away from the courthouse and administration building.

The horse escaped from the Prince George's Equestrian Center, where it was to perform at the Maryland Horse and Pony Show this week.

"It was a freak accident," said Anita Pesses, spokeswoman for the Maryland-National Capital Park and Planning Commission, which owns the equestrian center. "I've been here for 16 years, and I've never seen anything like this happen here."

Stevenson said he's known of some cows that have gotten loose from farms in the area but never a horse. "We've never had a fatality as a result of this," he said. "This is new."

Imwalle's family could not be reached for comment. Police said they do not know where she was headed when she was killed.

The horse was one of 450 that came to town Wednesday from about the mid-Atlantic region for the "100 jumper" show, in which the animals compete in various categories but do not race. Becky Doyle, event manager for the horse and pony show, would not release the name of the horse, his owner or any information about them.

"It's personal," she said.

Unlike some Prince George's communities that have become urbanized, Upper Marlboro still has agriculture, including tobacco farming, as part of its economy. But the town has been slowly transforming from a rural to an increasingly suburban area, mirroring other parts of the county outside the Beltway. It has a blend of farms, historic homes, upscale housing developments and government buildings.

The equestrian center rents space to horse shows, basketball tournaments and other events for revenue.

Doyle said the horses competing in the "100 jumper" show are either thoroughbreds or warm-bloods.

"When people hit a deer, they are surprised at the amount of damage it does to their car," said Mark Brady, spokesman for the Prince George's Fire/Emergency Medical Services Department. "This was larger than a deer. It did some destruction to the vehicle and the driver's compartment area."

Pesses said that when the horse escaped from the groomer and owner and bolted out of the stall, he was not wearing a bridle, which is unusual. She also said it is not usual practice to keep a barn gate open.

She said the horse ran the distance of "a couple of back yards" and onto the highway, with the owner and groomer chasing.

"They were very upset about the whole thing," Pesses said. "Clearly, there are emotional issues there."

Pesses said that the horse handlers did not violate any rules by leaving the gate open and the horse unbridled but that the center was evaluating whether to implement such regulations.

"We will be looking at this whole thing from an operational standpoint," she said.