ONE DAY WHILE driving home, Dennis Diaz saw an obviously injured parakeet lying on the side of the highway. He stopped his car, scooped up the half-dead bird, took it home and he and his wife, Linda, care nursed it back to health. "He's like that," his wife says. "You should see what he's brought home."

DAWN, EARLY JUNE a year ago: A filly and a dark brown colt circle the three-quarter-mile track at Dennis and Linda Diaz's Hunter Farm near Tampa. As the two horses gain speed to begin a training race, their hoofs cut cleanly into the deep dirt.

The filly is a speedster, a proven runner who knows her way around the track. The colt is untried, mostly untrained. "It will be good for that colt to get some dirt in his face," says farm manager Dave Hinkhaus as the horses reach their starting mark.

When the jockey kicks her ribs, the filly jumps to a quick lead, leaving the colt behind. Then the colt charges -- muscles taut, eyes bulging, hoofs beating in a synchronized gait. Soon the colt catches the filly, passes her and pulls ahead by a length, then another length, streaking across the finish.

Hinkhaus fumbles with his stopwatch: 36 and two-fifths of a second for three-eighths of a mile. He looks up. Then he looks at the watch again. He turns to Dennis Diaz, the colt's owner. "Boss," Hinkhaus bubbles with surprise, "that son of a bitch can run! He really can!"

LESS THAN A YEAR after his impromptu race with the filly on that little-known Florida farm, the colt named Spend a Buck took the honors at the 111th Kentucky Derby by running 51/4 lengths ahead of the pack, the biggest margin of victory in the race since 1946 and the third fastest time in Derby history.

Dennis Diaz had come almost by fate to the winner's circle at Churill Downs. About a year earlier, uncertain himself about Spend a Buck's potential, the owner had decided to sell the colt at auction. But the auctioneers declined: not good enough quality, they said. Although getting stuck with a horse that's hard to sell is not a new predicament in racing, Diaz was furious. He kicked the floor, kicked the barn, shouted and pouted. Then he decided to find out just what he was stuck with. When he found out, Diaz came to believe fervently in the horse.

The Derby proved to Diaz that he had done it, had taken his $12,500 underdog colt that the experts said didn't have what it takes and won anyway. And to Diaz the Derby proved he had been right about his staff, a group he carefully picked partly because they, like the horse, were also underdogs.

Since the Derby, Spend a Buck has gone on to win more money than any thoroughbred stallion in history, earning $2.6 million in a single race and a total of $3.9 million so far in his 13-race career.

And yet Spend Buck's accomplishments puzzle many. Rowe Harper, who sold the horse to Diaz, says: "There was nothing about that colt that was honestly outstanding . . . absolutely no signs that this horse was special."

"I'm afraid that I, as well as many other people, underestimated that colt," says Edward L. Bowen, managing editor of The Blood Horse, the bible of thoroughbred breeders.

It may have been terribly lucky for Dennis Diaz to have bought Spend a Buck in May 1983, and even luckier that he couldn't sell him when he wanted to, but the winning wasn't luck.

"Dennis has more or less picked underdogs to help all his life," says his wife. "He's always trying to make a silk purse out of a sow's ear. I think it has to do with his background. He knows what it's like not to have much of a chance."

An unknown trainer, an unknown groom and other obscure helpers were people Diaz felt would give their best to prove something. "Spend a Buck was my big shot," Diaz says. "But it was their big shot, too. That horse meant as much to them as he did to me . . . I could have hired big names, many people told me that I should. But I knew that big names had other horses.

"I only had Spend a Buck, and that's all they had, too. If he made it, we all did. They had to give me 100 percent."

DENNIS DIAZ has lived two very different lives: before Spend a Buck and after Spend a Buck. He found fortunes in both, but his first life was harder, and less happy.

Diaz's grandfather, Sacramento, emigrated here

from Spain in 1910 at the age of 15, earning enough eventually to start a dairy outside Tampa. Diaz's father, Sacramento Jr., grew up working on the farm, as did his sons, Dennis and Lesley.

"At 2 a.m. and 2 p.m., you milked the cows regardless of whether it was Christmas, New Year's or whether you were tired, sick or had a hangover," Diaz recalls.

He left the dairy for college, but dropped out after two years. He sold insurance and later fertilizer products. He worked hard, often 4 a.m. to midnight.

In 1965, at age 23, his father helped him buy a small insurance agency. Within 15 years he had 245 employes in a business that included real estate and construction interests. He was a millionaire.

But his success came at a price. Diaz had been married and divorced twice and his relationship with two children from his first marriage was, in his words, "rotten."

He was tired, burned out. "The business had gotten too big . . . I'm much better dealing with a small group of people that I like than managing an office."

He complained that the nature of the insurance business had changed: "Clients would say, 'Dennis, you've been our agent 10 years and you have done a nice job, but this other agency will write our insurance $10 cheaper so we are switching.' There was no loyalty anymore and you couldn't trust people anymore."

So in 1980, two years after his third marriage, at age 38, Diaz sold everything and retired. He went fishing, played the stock market and went to the race track.

His interest in horses came easily. He had a pony as a youngster he named Dynamite. He liked the slower-paced, baronial life style of horse breeders. A horse farm would be good for him, he decided, and a great place for him and Linda to raise her daughter Jenifer.

Diaz's second life seemed to be taking shape around the idea of horse breeding, but his business instincts told him to hold off awhile. "Everyone was getting into horse breeding at that time (1981-82) out of greed. They didn't have any love of the business or horses. They just wanted to make a lot of money and they thought if they bought a racehorse they would."

Diaz waited for the slide. "That is when I like to get into any business, when it is hurting, when everyone else is getting out."

He bought 50 acres of land in mid-1982, land his grandfather once had owned. It bordered a ranch owned by Elliott Fuentes, an acquaintance who had bred horses for 37 years. The men became friends and Diaz bought his first horse, a filly named Heated Rush, for $9,000 from Fuentes. In her first race, Heated Rush won $10,000. Diaz thought he had a magic touch. He was wrong. The horse didn't win another race for nearly two years.

JUST AS HE HAD DONE in his insurance business, Diaz mpped out a long-range strategy for horse breeding. He would breed mares, race one or two horses a year and slowly improve the bloodlines. Horse breeders follow a simple genetic philosophy: winners beget winners.

Diaz decided it might take him 10 years to breed horses that could compete in the nation's big-stake races, but he'd get there. "I didn't set out in this business to lose money," he says.

He began looking for stock and in the spring of 1983, a distant relative employed at a Kentucky bank called him about a horse sale: Rowe Harper had several good-quality yearlings and mares to sell, some cheap. Diaz studied their pedigrees. One mare, Belle de Jour, interested him. The horse had been sired by Speak John, a son of Prince John, who had won $212,818 racing. Harper also had one of the mare's foals for sale, a colt sired by Buckaroo, son of Buckpasser, a $1.4 million race winner. The horse's blood wasn't blue, but it was darker than red.

Diaz liked what he saw when he examined Belle de Jour in March 1983, but found the foal disappointing, smaller than the other yearlings and awkward. Diaz wasn't sure the colt was worth buying. But then he noticed something unusual: Whenever Diaz approached, the colt fled and ducked behind a bigger yearling.

"He was using the bigger horse as a bodyguard," says Diaz. "I liked that. It proved that he was smart." Diaz bought both horses.

"I had at least five other yearlings that were better -- stronger than that colt -- for sale," says Harper of the yearling that became Spend a Buck. "He didn't look like he would have won a race."

MEANWHILE, back at the farm, Diaz's luck was running bad. "I had bought a mare for $125,000 that was about to foal. She stepped in a hole, fractured her leg and delivered four weeks early," Diaz says. He raced the colt to the veterinarian's, nearly burning up the engine in his truck. "We got there, pulled the tailgate down and I lifted out the baby. The vets were coming with oxygen, but it just died in my arms."

Two weeks later, a promising filly worth $60,000 died of a virus. "We couldn't do a thing right for months," Diaz says. "We could have raced against cows and lost."

Fuentes told him to relax. "He said your luck always turns in horse breeding. This is a business of sheer joy and utter heartbreak where you get to experience low lows and high highs."

Diaz decided to improve his cash flow by selling the flighty colt he had bought from Harper. He notified the operators of the Ocala Select 2-year-old sale and set a price of $30,000 for Spend a Buck. When the horse wasn't accepted for the sale, Linda Diaz remembers her husband "kicking a stall" at the time and saying, "I'd sell that damn colt in a second if someone would take him."

At the time, the only bright spot at Diaz's stables was the arrival of Camillo Michael Gambolati as trainer. Diaz had met Gambolati when they happened to sit next to each other at a horse sale.

"I was impressed by his intelligence and his style. Too many trainers try to push horses instead of listening to them," Diaz said. "Horses don't lie to you. If you're smart enough to lis- ten, they'll tell you what kind of condition they're in, what kind of ability they have. Cammy understands that."

Gambolati, 35, was not a hot prospect in the racing community. Since graduating from a Tampa-area college, he had done a number of things, from owning a laundromat to being statistician for the Tampa Bay Buccaneers. His passion, however, was horse racing.

At Hunter Farm, Gambolati had fewer than 10 horses to work with including the ill-tempered Spend a Buck that stable hands called Harvey Wallbanger because he tried to knock them against the walls of his stall.

In August 1984, Gambolati declared the colt ready to compete. In his first race, Spend a Buck came in second behind a horse named Smile. Two weeks later, Spend a Buck won. He then took another first place in the Miller High Life Cradle race, winning by a whopping 15 lengths.

"We knew the horse was fast," says Diaz, "but we still didn't know how fast."

Diaz entered Spend a Buck in the $622,200 Arlington-Washington Futurity in Chicago, the horse's first race against top national talent. "A number of people told me I was nuts for taking him out of Florida," says Diaz. "They said he'd never make it."

A promising colt named Proudest Hour was favored to win the Chicago race, but he was put away early in the race. When the horses reached the top of the stretch, Spend a Buck surged, winning by a half-length.

Diaz began to dream. "I knew this horse was really special then," he said.

A few days after the race, a representative for jockey Angel Cordero Jr., a two-time Kentucky Derby winner, telephoned Diaz. "When I was first contacted by Angel's people, I said, 'No, absolutely not! I'm not interested in switching jockeys.' jockey, had ridden Spend a Buck from obscurity to his Chicago win. "But then I decided that it was inevitable," Diaz said. "It would be easier now than later."

Diaz dumped Hussey and hired Cordero. It was the "hardest thing that I've ever done," Diaz says. "I love that guy." Diaz keeps a photo of Hussey in his office.

In his next race, the Young America at the Meadowlands, Spend a Buck led most of the way but lost at the wire. He then finished third in the Breeder's Cup held Nov. 10. Diaz was happy -- his horse had run well, losing to strong contenders, Chief's Crown and Tank's Prospect, two horses that would be favored in the Kentucky Derby.

The day after the race, Diaz went to see Spend a Buck. "He came out lame. I couldn't believe it," Diaz says. "I thought I had blown it."

A bone in Spend a Buck's right front knee had been chipped during the race. Diaz called in a surgeon who specialized in the use of arthroscopic surgery on horses. The operation took 12 minutes. Three months later, Spend a Buck was galloping.

Diaz chose the Bay Shore, a seven-furlong race at Aqueduct, for Spend a Buck's return to racing. "We were pressed for time if we wanted to make the Derby, but we didn't want to hurt the horse." He told Cordero to give the colt a good workout, to win if possible, but not to "do anything to hurt the horse."

Spend a Buck finished third and Diaz was exhilarated: "He was okay." Handicappers were less enthusiastic. They saw Spend a Buck as yesterday's news. Even Cordero began saying he might bypass the Kentucky Derby to ride another horse at another track on Derby day.

Then Spend a Buck won spectacularly. In April, the horse won two races at Cherry Hill, N.J., and came within two seconds of breaking the world record set by the mighty Secretariat for a 1!- mile race.

Spend a Buck had earned his spot at the Kentucky Derby, but he was clearly an underdog: some handicappers felt Diaz was bringing his horse back too soon after surgery; others said Spend a Buck liked to break fast from the tarting gate and would have trouble maintaining his speed against a field of horses considered the most competitive in recent Derby history. Some still maintained that Spend a Buck couldn't cut it against top competition.

Diaz was unflappable. "If Spend a Buck gets a good start," he predicted, "no one will catch him."

W.S. Farish, a wealthy Kentucky horse breeder, quickly put Diaz's faith to a test, asking the owner to sell him a 50 percent interest in Spend a Buck for a reported $4 million.

Diaz, Linda, Gambolati and Peter Hall, Spend a Buck's veterinarian, held a meeting. If the horse won the Derby, his worth would increase by millions, but if he lost, they might not be able to equal Farish's offer. Diaz called for a vote: "Linda said sell, Cam said sell, and the vet said sell. I was the only one against selling the colt before the Derby.

"I'd promised to be democratic, but I'd retained the right to set the conditions of the sale." He made them so restrictive that Farish declined. "I had faith in that horse. I was going to stay with him."

LINDA DIAZ'S favorite photograph from the Kentucky Derby shows her husband holding their adopted son, Elliott, in his arms while she holds the gold trophy aloft with daughter Jenifer beside her.

"We planned it that way," says Linda. "We wanted to make a statement to the Derby people and the world that this was our family and we had done it together -- as a team."

Elliott, now 21 months old, gave Diaz a second shot at being a father. He arrived, like Spend a Buck, as an underdog, brought by fate.

In the summer of 1983, an attorney offered the Diazes a Hispanic baby for adoption, a child doctors believed would face medical problems as he grew older. Twenty other families had politely declined -- the Diazes said yes.

"This child needed me and I needed him," says Dennis Diaz. "Look, I'm 42 years old," Diaz says. "I've got a lot of miles under me. I have two children by a previous marriage that I'm not close to. For a lot of reasons that marriage failed. But I'm more mature now. I think I can appreciate a son . . . this is a second chance for me."

Elliott's arrival brought pain as well as joy. Linda is blond and blue-eyed. Elliott is dark and kinky-haired. "I enrolled us in a mom and baby gym class," Linda says. "When a woman there saw Elliott she asked me pointedly if I was married to a black man." A few days later, Jenifer came home in tears after school because a classmate who had seen Elliott told her that he "must have been a mistake."

"I had never encountered prejudice before," Linda says. "I have tried to be nice about it, to educate people, but some people are just stupid."

ON A MID-MAY morning at Hunter Farm Diaz takes calls from the media about the Preakness, the second jewel of racing's Triple Crown. Diaz bypassed the race because he wanted his horse to run in the Jersey Derby. If Spend a Buck won that race, he would get the $600,000 purse and a $2 million bonus because of his prior wins at the track and his Kentucky Derby victory. Diaz's decision was hotly criticized because he had abandoned tradition for money. He didn't care.

"It was pure economics," Diaz says. "Look, people told me that my horse would be worth $20 to $40 million more if he won the Triple Crown, but I'm not that damn greedy. I've already won more than I ever thought we would and $2.6 million is a hell of a lot of money."

Many handicappers said Spend a Buck wasn't good enough to win the Triple Crown.

"You try to keep a perspective," Diaz says. "In this business, believe me, you are the man of the moment. You are as good as your last race. If this horse gets beat a couple of times, then no one is going to want to talk to us . . . I know that."

He is late to lunch. Linda is waiting at the house with Walter Farley, author of The Black Stallion, and his illustrator, Angie Draper, who wants to paint Spend a Buck's official portrait.

"I really don't enjoy this," Diaz says of the attention. "I'm more comfortable working in the stables."

Success forces change. Spend a Buck is guarded around the clock. Elliott and Jenifer must be protected. Life insurance premiums for the horse exceed $900,000 a year. Linda has hired a fashion coordinator.

"I have to be careful," says Diaz as he rides in a golf cart from his office at the farm to his house. "I don't want the same thing that happened to my first marriage to happen again."

Like so many other important people in Diaz's life, Linda, too, was in limbo when they met. She had married young and divorced after seven years. Her first husband "wanted a very typical housewife. There is nothing wrong with that . . . I am just not one of those people. I had to leave that relationship in order to self-actualize."

A registered nurse, she had returned to college in Tampa to earn her bachelor's degree when she met Diaz. She liked his ambition. "Most people have ambition," she says. "What makes Dennis and me different is that we are willing to devote whatever it takes to reaching our final goal, absolutely whatever it takes."

She went to work in Diaz's insurance office after they were first married, but it didn't work. "We are too strong-willed. It's all, 'I'm right and you're wrong.' There is no gray. But after the screaming is over, we usually laugh and find gray."

She worked in a figure salon, held other jobs and then returned to college for a master's degree in business. "I didn't know what I wanted to be when I grew up."

Spend a Buck changed that. Now Linda helps handle Diaz's schedule and run the farm, and she takes care of Elliott and Jenifer. "I may not be mucking stalls," she says, "but believe me, I've worked hard to help this happen."

THE NEXT MORNING, Diaz is nervously waiting to talk to Gambolati by telephone. He lights a Marlboro 100 while smoke from the last cigarette curls up from the kitchen ashtray.

Jockey Cordero has a riding conflict on the day of the Jersey Derby. He wants time to see if he can somehow keep both riding dates -- just as he did on Derby day when he left Kentucky immediately to ride in another race.

"Angel is trying to have his cake and eat it, too," Diaz says. He replaces Cordero with another jockey dispassionately.

He is worried about the race, about winning the $2.6 million. "It's the last thing I think of at night and the first thing in the morning."

The race will open the door to a much bigger fortune. Spend a Buck's real value will come when Diaz sells shares in him as a stud, a process called syndication. Each shareowner will get to mate a mare to Spend a Buck. The great Secretariat was syndicated for $6.08 million in 1973; Seattle Slew went for $12 million in 1978; Spectacular Bid was syndicated for $22 million in 1980; Devil's Bag, $36 million in 1983. There are differing figures for what Spend a Buck could bring at stud, with a conservative estimate being $10 to $12 million. Diaz thinks it will be at least $20 million. "That's more money than I ever dreamed I would earn in a lifetime," Diaz says, lighting another cigarette. "It could affect our children's future and our children's children's future and their children's children's future."

"When I was young," says Linda, "I always wanted to be rich and famous, but this . . . " She looks upward and smiles.

Although they believe their success has more to do with hard work than luck, the Diazes are at times uneasy about it. Linda has turned to astrology for explanations. Diaz has looked inward. "This is a fantasy world we are living in right now, a 'Dallas' soap opera, but it is something we have to do," Diaz says.

"Why me?" he wonders. "I've thought about it a lot. Who knows? Some of it is the luck of the draw."

He pauses: he doesn't seem to believe that. "I think everyone gets a shot in life. If you work hard, and believe me, I have worked hard . . . then to some degree you earn it. I mean, we took the risks, we put our money where our mouth was."

ON MAY 30, Spend a Buck won the $2.6 million Jersey Derby in the closest race of his career. He had the early lead, but was passed twice by Creme Fraiche, the colt that later won the Belmont Stakes. Spend a Buck came back and won by a neck at the wire.

"He proved at the Kentucky Derby that he could run," Diaz said later. "But this was the race where he proved he had heart. He wasn't going to let anyone beat him."

Underdogs understand that sort of thinking.