When Mark joined the Army during the Vietnam era, he intended to learn a skill with which he could earn a living in civilian life. This he did -- though not in quite the way he expected.

It would have been hard to imagine that flying a helicopter gunship over the jungles would prepare him for playing the horses in later life.

Mark became a commercial pilot when he was discharged from the Army, but after a few years he was grounded by a physical disability. He contemplated his alternatives and concluded that what he really wanted to be was a gambler.

He plunged into a serious study of handicapping; he even had charts of speed figures blown up to giant size so he could cover the walls and ceilings of his room and be able to study them during all of his waking hours.

As his study progressed, he said, "What I really wanted to do was get into the horses' heads. The only way to do that was to get into the trainers' heads. One day I thought, 'Wait a minute! I know a way.' "

He remembered his Vietnam days when his helicopter would make surveillance flights, using the most sophisticated sound-detecting equipment to look for Vietcong in the jungle. Mark understood enough about the technology of eavesdropping to envision some practical applications of it in his new area of endeavor.

"I went to an electronics store that sells listening equipment," he said, "and spent $350 for equipment that would fit in the arm of my jacket. It's a boom microphone with wires leading to a main box. What it picks up is fed into a recorder and into a small microphone that I wear in my ear like a hearing aid."

Armed with this equipment, he started going to the tracks in Maryland and West Virginia and listening to private conversations. He would go to the paddock before races and listen to what the trainers were telling the jockeys. He would hang around areas where trainers and other track insiders would congregate and listen for assorted information.

One night at Charles Town he got fragments of a conversation between two trainers sitting in the grandstand: "Did you clock him?" . . . "We've got a good bet coming" . . . "I'll see you at McDonald's Friday at 7:30 and I'll let you know what we have."

Mark was at McDonald's at 7:30 on Friday, listening to the men banter with each other until one said: "I don't want your buddies to load up on this, so let's just keep it between us. Just go 7-8, 8-7 in the exacta."

Mark loaded up, and the 7-8 exacta combination paid $156, giving him his first big score as a race track spy.

Not all of his eavesdropping is so dramatic, but Mark said that the relatively routine remarks of a trainer to a jockey in the paddock frequently provide the key to horses' chances. When a horse is well-meant, the trainer will let the jockey know it (e.g., "We've got a good play on this horse and you'll make some money if you win."). Of course, many trainers and insiders are just as fallible in their judgments as anybody else, but Mark says that two of the targets of his surveillance have extraordinary batting averages -- one West Virginia trainer, the other a wise guy who makes occasional appearances at the Maryland tracks.

Mark not only knows plenty about the betting habits of racing professionals but also about their marital, extramarital and financial affairs. Maryland horsemen who are planning any covert activities might be advised to do so in hushed tones.