Dennis Ayres was not born on a horse, but he has more than made up for it. He was raised on a horse, he works on a horse and he plays on a horse. He buys horses, sells horses, trains them and shoes them. He even likes the blamed critters.

"It's all I know," says Ayres. "It's all my father knew and all my grandfather knew. It's a way of life."

A jockey might say something of that sort. Or a cowboy, a rancher, a trainer or a stunt man. But Dennis Ayres is a policeman. Washington's clippity cop. Our answer to Sgt. Preston and that Yukon gang of his. Our living reminder that policemen didn't always drive air-conditioned Plymouths.

Ayres is a U.S. Park Police sergeant and the department's director of equitation training. That not only puts him in direct charge of 59 horses - their feeding, housing and training - but it means he trains every officer who gets on one.

It is a long a tough job, and it is something less than dignified. Every night when he goes home, Ayres must change his clothes in the basement and scrape his boots to remove the . . . well, call it the distinctive horse odor.

But nothing interrupts the love affair between this man and a horse.

Dennis Ayres moonlights as a blacksmith and a free-lance horse psychiatrist for two Northern Virginia hunt clubs. He is captain of the Park Police horse team. He holds a horseshoeing certificate from Michigan State University. And he is the leader of the pack whenever mounted Park Police are called into emergencies.

Enough? not for Ayres. How does he spend a day off? On horseback, of course. And how does he spend a vacation? In January, he went to Ireland for a week - where he spent the whole time hunting on horseback.

The only quirk is that Dennis Ayres does not own a horse. "Could never justify it," he says.

And now he has a new challenge. Mounted police patrols are making a big comeback around the country, and Ayres has been assigned to train delegates from all interested departments. Fourteen departments have been through Ayres' course in the last five years.

So it was very much in character to find him perched one recent afternoon, in his black jodhpurs, high atop the practice ring of the Park Police horse barn in Rock Creek Park. In front of him, 13 riders in the police uniforms of Albuquerque, Atlanta and Charleston, S.C., kept shuffling past. They were in something less than total control of their mounts.

"Get those hands down!" Ayres would cry, or "Lead him, lead him!" And almost every time, the hands would stay up, or horse would lead human. "They've got a lot to learn," Ayres sighed, sounding like a football coach whose players are mostly freshmen.

But once they learn it. Ayres feels the police benefits will be great.

"On a horse, you're a mobile footman," he said. "You're using the horse as a mobile tool. And all of a sudden you know who lives in the third house from the corner again."

Not only that, but an officer on horseback commands a kind of respect - ad emits a set of vibrations - that walking or riding police just don't.

"The guy on the horse is a good guy," said Ayres. "I've been on horseback in every major demonstration here in the last 20 years. Every time, you can hear the crowd hollering, 'Don't hit the horse!' They can be hostile and nasty and they don't want to shake hands with policemen. But they always want to pet the horse."

How the horse feels about all this is something else again. HIs natural instinct is to buck, or run. So horse training in Ayres' shop is a rigorous affair.

"We get horses on a 30-day contact. We've only got that long to find out if he has the temperament," Ayres said. "When you get 40 sreaming kids running up to a horse and he stands, that's fine. But the minute you get a horse in somebody's baby carriage, you've got problems."

There literally was a horse in Ayres' baby carriage. He grew up in Georgetown, within an easy canter of the Edgewater Stable that was razed to make way for the Kennedy Center.

"My father went through law school breaking horses for Thompson's Diary," Ayres recalls. "The dairies all used horses in those days to deliver the milk. Before I could play football of baseball, I had to come home and take care of a horse."

Ayres' grandfather had set the pattern a generation before. He was brought over from Ireland by a consortium of railroads. His job was to shoe the horses and mules that were used to build a spur near Richmond. He moved to Washington just after the turn of the century.

His grandson is still very much here, supervising mounted patrols on the Mall, the C&O Canal and along the Rock Creek Park trails. It is a proud beat, and one that requires constant riding. "Our guys never tie up and go check out the newest movie on 14th Street," says the boss.

He pauses to shout at an Albuquerque officer. "Hands down! Heels down! Don't cut that corner on me." And then he ruminates.

"This is a tough life. You've got to want it. Here you are, spending more time with a horse than with your family.

"But it's the elite. At least we like to think it is. I've never had a department trained here that didn't go back home and grow.

"Sure, I'm tough. But I've got to be. My dad always told me a horse was beast of burden. Make a pet out of him, and you've got problems.

"There's a lot of satisfaction in getting a horse to do what you want. Like I said, it's a way of life."