Robert Brennan, the New Jersey stockbroker who has spent $12 million buying thoroughbreds during the past year, collected the first big dividend on his investments today.

His colt, Hurry Up Blue, won the rich and prestigious Gulfstream Park Handicap and made Brennan's purchase look like a bargain.

Last summer, during his whirlwind spending spree, Brennan bought Hurry Up Blue as part of a four-horse package for $875,000. The colt's credentials were modest, but he blossomed when he came to Florida this winter.

Two weeks ago he upset Tunerup, the highly regarded stakes horse, in the Donn Handicap. Today the crowd here dismissed that result and made Tunerup the favorite again. They were decisively wrong.

With no speedballs in the field, neither jockey Carlos Lopez on Hurry Up Blue nor Don Brumfield on Tunerup wanted to let the other steal an early advantage. The two hustled to get the favorable position early and then galloped abreast of each other at a steady pace: a quarter mile in 24 seconds, a half in :48, three-quarters of a mile in 1:12.

Both horses should have had plenty of energy left after such minimal exertion. But Tunerup, who was sold for $1 million last fall, started to weaken on the turn, and Hurry Up Blue opened a clear lead turning into the stretch.

In the last 16th of a mile, Hurry Up Blue started to weaken, too, but he managed to hold off the fast finish of 82-to-1 shot Yosi Boy by a length. Imperial Dilemma was third, another length behind.

Hurry Up Blue paid $6.60, $4.40 and $3.80 to his backers while earning $114,887 for his owner. The colt had covered the 1 1/4 miles in a slow 2:03 1/5 on a fast track, evidence that this was not a particularly formidable group of older horses.

But trainer Reynaldo Nobles believes that Hurry Up Blue has a bright future in front of him. "I'm going to show more with this horse," he said. "This is only the beginning." He indicated that he would probably ship Hurry Up Blue to Bowie for the John B. Campbell Handicap.