For sheer thrills and chills, the United States Olympic Track and Field Trials is surely one of America's greatest sporting spectacles. It is an all-or-nothing venture in which fourth place is no place, in which, as Olympic steeplechaser Henry Marsh noted, "a lot of dreams are made or dashed."

And this year's Olympic trials, which will begin tomorrow in Indianapolis, promises to be something special, even by its own standards: Mary Slaney's comeback. Marsh's last hurrah. Edwin Moses' ageless mastery. Jackie Joyner-Kersee's rising stardom. Carl Lewis' second quest for Olympic immortality.It will be a memorable Trials. And yet already you can hear the talk start anew. Is the Olympic trials the best way to choose an Olympic team? It's an old argument, and it began early this spring when Joan Benoit Samuelson, the defending Olympic marathon champion, missed the U.S. Olympic Marathon Trials because of injury. Then last week Greg Foster, the No. 1-ranked hurdler in the world, went down in a heap during a workout and shattered his left arm, seriously jeopardizing his chances of surviving the trials.

By the time the Olympics arrive in September, Foster and Samuelson might very well be the best high hurdler and female marathoner, respectively, in the world, and not be in the Olympic Games in Seoul, South Korea. But there it is: Samuelson wasn't ready for the Trials, and now Foster, if he can compete at all, might be unable to carry a cast over 10 hurdles fast enough to crack the top three. That's what it takes to make the U.S. Olympic Team, and there never have been any exceptions.

"I've been an advocate for changing our procedure for some time," says Marsh, an Olympic steeplechaser from Bountiful who is chairman of the athletes' advisory council to the U.S. Olympic Committee. "We don't protect our top people. The Foster situation exemplifies that best."

No other country in the world picks its Olympic team the way the U.S. does. Other countries allow varying degrees of subjectivity and flexibility in their selection process, but not the U.S. Thus, when Ben Johnson, Canada's sensational world 100-meter dash champion and world record holder, pulled up lame early this summer, there was no danger that he wouldn't make his country's Olympic team. If he were an American, Johnson might be forced to watch the Olympics on TV.

An Olympic Trials is purely an American concept. "It reflects our desire not to be political," says Bob Wood, a track afficionado and agent of 26 Olympians. "It is the fairest system, especially given the size of the country and the magnitude of the talent."

Indeed, the deep well of American talent, with the aid of the trials system, has produced some wonderful surprise Olympic heroes, such as Billy Mills and Dave Wottle. But the hazards remain. If Lewis missed the Olympic Trials long jump competition because he got stuck in a traffic jam, or if Moses hit a hurdle and fell, they would miss the Games. Never mind that they have consistently dominated their sport for years.

There are other ways to pick a team, of course. Says Marsh, "Give the highest (U.S.) finisher in the World Championships (held the year before the Olympics) an automatic berth on the Olympic team, provided he shows a certain level of fitness. Or, give the national champion an automatic berth. Anything, just so it's not a one-shot deal."

On the other hand, that is precisely the meet's charm. As Bob Wischnia wrote in Runner's World, the very cold impartiality of the meet is also the beauty of the meet. This is not the NBA playoffs, in which second (third, fourth and fifth) chances run amok.

"Because of the current system, the Olympic trials is the most exciting meet there is," says Marsh. "It's do or die."

"There's immense pressure," says Doug Padilla, winner of the '84 Olympic Trials 5,000-meter run. "I felt more pressure at the Olympic Trials than in the Olympics. (Miler) Steve Scott says the same thing."

"It's a traumatic experience," says Ed Eyestone, who finished second in the marathon trials and competed in the '84 track trials. "You know those stress ratings you see, where death of a spouse and loss of a job is worth so many points? Well, the Olympic Trials is worth about 120 points on the stress meter."

From nine stressed out days (July 15-23) will come the U.S. Olympic team. Among the meet's leading attractions will be Lewis, who won four gold medals but not public affection in '84, thanks to his own calculated commercialism and media ploys. The way has been cleared for Lewis to try again for four gold medals now that the IOC has agreed to his request to separate the long jump and 200-meter dash to allow him time to rest between the events. But first he must turn back the promising young American talents of Joe DeLoach and Lorenzo Daniel, among others. The U.S. sprints are so deep that even world champion Calvin Smith will be hard-pressed to make the team in the 200-meter dash.

Lewis, 27, says this will be his final Olympics. It also will probably be the final trials for Marsh, Moses and Scott, some of track's most enduring stars. Marsh, the best steeplechaser this country has ever produced, will retire at the end of the year. Scott, 32, is winding up his brilliant miling career; sadly for America, there is no one on the horizon to replace him. Moses, 32, stubbornly talks of returning for the '92 Games and, given his 12-year dominance of the intermediate hurdles, perhaps it is not so far-fetched; then again, lately he's been hearing the footsteps of Danny Harris in the homestretch.

The trials' leading ladies are also a veteran set: Slaney, Evelyn Ashford and Samuelson, who have all endured child birth and numerous injuries since the last Olympiad. Slaney's only obstacles are her own fragile limbs, which have forced her to sit out most of the past two seasons. Ashford, the world record holder at 100 meters, hopes to repeat her gold medal performance of '84, if only her tender hamstrings will hold out. Unable to compete in the marathon, Samuelson will try to make the team in the 10,000. This redoubtable trio, however, might be forced to take a back seat to Joyner-Kersee, a modern Babe Didrikson who could win the hurdles and long jump after winning the seven-event heptathlon, if the schedule allowed it.