The president of Virginia's Board of Veterinary Medicine, one of the nation's top equine veterinarians, serves clients ranging from President Reagan to Jacqueline Onassis.

Dr. John Mayo of Middleburg grew up on a horse farm in Hanover County and said he knew he wanted to be a veterinarian, even in high school.

In a business where breeding means everything, Mayo's own bloodline stands out in Virginia. His ancestor, William Mayo, laid out Richmond's original street plan in the 1600s and a city bridge is named for the Mayos.

His parents were noted horse trainers and his father, Newton Mayo, is one of the nation's foremost equine artists. One brother operates a gallery in Richmond.

He studied at Virginia Tech and received a degree in veterinary medicine from the University of Georgia.

Mayo is the senior partner in Mayo and Roffe Equine Clinic Inc., the largest private equine practice in Virginia.

Spartan offices are housed in a weathered frame building wedged among the stables adjoining the Middleburg Training Track.

His dusty office is decorated with several of his father's paintings and an autographed photo of Reagan sitting tall in the saddle.

Asked if he has to travel to Washington to care for the horses Reagan rides in the city's Rock Creek Park, Mayo answered, "No, they bring them down here."

He routinely visits the sprawling Fauquier County farms of multimillionaire Paul Mellon, a noted thoroughbred owner and philanthropist, and Sen. John W. Warner (R-Va.), who also likes to ride.

Mayo first cared for Jacqueline Onassis' horses when the Kennedys owned a Virginia farm during the presidential years. He cares for the horse she rides when she visits the Mellon estate.

One of the big thoroughbred breeding operations that relies on Mayo's professional know-how is Buckland Farms near Gainesville, home of 1981 Kentucky Derby and Preakness winner Pleasant Colony.

Mayo became affiliated with the Olympics and the Pan American Games through his interest in "Three-Day Events," an international equestrian sport featuring the precision gaits of dressage, stadium jumping and hazardous run-and jump cross-country exercises during a grueling three days.

As one of only a handful of three-day-event veterinarians in the United States approved by the Federation of Equestrian Internationale, Mayo was honored with a veterinary post with the 1984 Olympics in Los Angeles.

Mayo said he examined horses midway throught the event to determine if they were physically able to continue.

"It's a pretty big responsibility to make that decision. A horse can drop dead from exhaustion and we're talking about some pretty valuable animals," he said.

Mayo will be back in the "Vet Box" again Aug. 8 through 10 at the Pan American Games in Indianapolis.

Mayo, who regularly works 12-hour days, puts as much as 60,000 miles a year on his mud-plastered blue pickup.

The truckbed is filled with a white "Porta Vet" unit containing refrigerated medicines, a portable X-ray machine, splints, surgical kit and just about anything else an ailing horse might need.

Mayo doesn't ride as much as he used to, but has participated in riding trips that his wife Sharon has organized in Africa and New Zealand.

"I'd rather hunt and fish than ride," Mayo said. He regularly does both in wilderness areas of Alaska, Labrador and Newfoundland. Scuba diving among the world's more remote barrier reefs is another of his hobbies.

"No matter where I go, however, I'm always glad to return to Virginia," he said. "It's not only God's country, it's horse country as well."