Laurel Race Course might conduct the following experiment as the ultimate test of the popularity of jockey Kent Desormeaux.

The track management could go to the National Zoo and ask to borrow a hippopotamus for the afternoon. The animal would be entered in the feature race at Laurel, and Desormeaux would be listed as the rider. What odds would the hippo be?

My guess would be 5 to 1, tops.

Enough horseplayers would reason: "If Desormeaux's agent took this mount, the hippo must have some hidden form. Besides, there's a lot of speed in the race, and the hippo looks like the kind of stretch-runner that the kid can bring from off the pace."

Actually, I might bet him myself at 5 to 1.

The sensational 17-year-old has become such a force in Maryland racing that horseplayers must reckon with his influence in almost every race, every day. His dominance is extraordinary and, in many ways, confounding.

Ordinarily, the jockey plays a relatively minor role in a serious handicapper's calculations. It's the horse that counts. Once the handicapper has picked his horse, his degree of enthusiasm may then be swayed by the competence of the jockey. If the rider is totally incompetent, a bettor might choose to eliminate the horse and look for another choice. But the jockey is rarely a central part of the decision-making process -- except in a few rare cases.

When Steve Cauthen was a phenomenal apprentice in New York, his mounts would win for no apparent reason other than the fact that he was on them. Jose Santos seemed to have similar mystical powers when he launched his career in Florida. Pat Day is such a standout at the midwestern tracks where he rides that he often gets his pick of the best three or four mounts in a race and his choice always has to be respected -- even if the horse doesn't seem to figure.

Desormeaux has this kind of dominating presence. He not only has the first call to ride for many of the top stables in Maryland, but he has so much talent that he will often win when he is on only the second- or third-best horse in a race. The 10th race last Tuesday was a case in point.

His mount, Rock Candy Girl, was nothing special. Her rival Win 'N Poker had much better figures, and another member of the field, Spillway, was the only horse in the race was early speed.

Spillway stumbled coming out of the gate. Win 'N Poker got trapped on the rail, the worst part of the track. And Desormeaux delivered a flawless performance to win aboard Rock Candy Girl.

She paid $4.60 -- and that payoff demonstrates why the Desormeaux Factor is driving Maryland horseplayers crazy. Even the mediocre horses he rides go off at short prices, and the odds on his clearly superior mounts are often unbettably inadequate.

Racing fans who recognized his skills a few months ago might have made good money by following him slavishly; a bet on every horse he rode at Laurel in the fall would have produced a profit. But now that the teen-ager has won the national riding title and the Eclipse Award as 1987's top apprentice, everybody has jumped on his bandwagon.

An approximation of my proposed hippopotamus experiment was actually conducted at Laurel Jan. 5. The kid wound up with the mount on a truly impossible horse. Gold Exchange's only career victory had come by a nose in a cheap maiden claiming race; when he ran in allowance company, he had finished eighth. He had been laid off five months since that race, and now was entered back in the same allowance class where he had been drubbed at odds of 49 to 1. If he had been 99 to 1, nobody would have done a double-take.

With Desormeaux, he was 5 to 1 -- and didn't get beaten too badly.

If the public's support of Desormeaux has been enthusiastic in recent weeks, it escalated to almost insane proportions Sunday. In the first race he was riding a filly, Hold Your Reply, who might have deserved to be a tepid favorite but nevertheless had won only one of her last 24 starts in cheap claiming company. She went off at an utterly ridiculous 3 to 5 -- and lost.

Desormeaux rode a colt named Proud Buck who had finished next to last in his only Maryland appearance and was bet down a to a ridiculous 4 to 5 -- and lost. He rode Band Leader, who merely looked like one of the contenders in a competitive allowance field, but went off at even money -- and won.

There is almost nothing a serious bettor at Laurel can do but try to beat Desormeaux, knowing that even the greatest of jockeys is going to lose 75 percent of the time. But we all know he's going to burn us time and time again when he wins on horses -- or hippopotami -- who don't figure. It's a classic case of being damned if you do and damned if you don't.