Sitting on the top shelf of Mark McGwire's locker, next to a can of Popeye spinach and packs of sugarless gum, is a brown bottle labeled Androstenedione.

For more than a year, McGwire says, he has been using the testosterone-producing pill, which is perfectly legal in baseball but banned in the NFL, Olympics and the NCAA.

No one suggests that McGwire wouldn't be closing in on Roger Maris' home run record without the over-the-counter drug. After all, he hit 49 homers without it as a rookie in 1987, and more than 50 each of the past two seasons.

But the drug's ability to raise levels of the male hormone, which builds lean muscle mass and promotes recovery after injury, is seen outside baseball as cheating and potentially dangerous.

``Everything I've done is natural. Everybody that I know in the game of baseball uses the same stuff I use,'' said McGwire, who also takes the popular muscle-builder Creatine, an amino acid powder.

However, many other players insist they do not take Androstenedione (pronounced Andro-steen'-die-own), although the use of other supplements is common.

Sammy Sosa, close to McGwire in the homer chase, uses Creatine after games to keep up his weight and strength. For energy before games he takes the Chinese herb ginseng.

But Sosa said he doesn't use Androstenedione or any other testosterone booster. Nor does Boston slugger Mo Vaughn.

``Anything illegal is definitely wrong,'' Vaughn said. ``But if you get something over the counter and legal, guys in that power-hitter position are going to use them. Strength is the key to maintaining and gaining endurance for 162 games. The pitchers keep getting bigger and stronger.''

Andres Galarraga, Atlanta's top home run hitter, said he would be ``scared'' to take a drug like Androstenedione.

``I do my weight (lifting) and take my vitamins. That's it,'' he said. ``You have to be careful what you take. It could cause secondary problems with your body.''

Shot putter Randy Barnes, the 1996 Olympic gold medalist and world record-holder, recently drew a lifetime ban for using Androstenedione. Barnes claimed he wasn't told about the ban until after his out-of-competition drug test on April 1. Barnes is appealing the decision.

Baseball bans only illegal drugs as does the NBA, and the reason in both cases has nothing to do with competitive fairness or health. The players associations and management in both sports simply haven't agreed on ways of dealing with the issue.

``Obviously, if there's more research and it's shown that it's harmful, we'll make people aware,'' baseball spokesman Rich Levin said of Androstenedione.

Numerous studies suggest there are dangers associated with drugs that raise testosterone levels _ even if there isn't much research specifically on Androstenedione.

``It's just a fluke of the law that this is totally unstudied,'' said Dr. John Lombardo of Ohio State, the NFL's adviser on steroids. ``There are no adverse-effect studies. There are no efficacy studies. Because the people who produce it never had to do them, thanks to the (federal) supplement act of 1994. Androstenedione is no different than taking testosterone.

``Androstenedione is a steroid,'' he said. ``It has anabolic qualities. Therefore it is an anabolic steroid.''

Anabolic steroids have been associated with potentially fatal side effects, including heart attacks, cancers, liver dysfunction, and severe disorders of mood and mental function.

``You can't even buy testosterone with a regular prescription,'' said Dr. Gary I. Wadler, an expert in supplement use and assistant professor of medicine at Cornell University Medical College. ``You have to get a triplicate prescription. It's a controlled substance by an act of Congress. The schizophrenia of all this is, product A, which is over the counter, becomes product B, which is a controlled substance.''

Creatine, which the 34-year-old McGwire believes helps him recover faster from daily weightlifting, is purported to increase muscle energy and mass. Long-term effects of the powder are unknown. It has been known to lead to muscle tears and cramps due to dehydration.

``I've been using Creatine for about four years,'' said the 6-foot-5, 245-pound McGwire, who played for the U.S. baseball team at the 1984 Olympics. ``It's a good thing. It helps strength. It helps recovery.

``I think Creatine is getting a bad rap now because people abuse it,'' he said. ``That's the problem. It says to take one to two scoops a day. People started taking 15 or 20. If you abuse anything you're going to hurt yourself. If you just use common sense, there's absolutely nothing wrong with it. It's a form of eating red meat.''

Chicago Cubs trainer David Tumbas said he doesn't recommend Creatine but doesn't tell players not to take it. He asked the players in spring training if they were using it or similar supplements, and about 10 said they were. He added, though, that he believes no one on the Cubs is taking Androstenedione.

``Our belief is still rest, nutrition, plenty of hydration and exercise are all you need,'' Tumbas said.

The IOC added Androstenedione to its lengthy banned list last December after it found the pills and various steroids being hawked on the Internet by a company called Price's Power International of Newport News, Va. The company, which offered the product for $49.95 a bottle and gave tips on how to avoid detection, claimed Androstenedione helps build lean muscle mass ``faster than ever imagined.''

But that's hardly the only place where ``Andro,'' as it is popularly called, is available. Great Earth Vitamin stores, a chain of 138 franchises in 23 states, sell the drug over the counter and by mail order. It is bundled with several supplements in a packet called ``Andro-Flav Stack.''

``It's very popular,'' said Andrew Fischman, director of marketing for the Hicksville, N.Y.,-based chain. ``The primary target of it is the 18- to 35-year-old muscle-head.

``If you can support your body's natural ability to produce testosterone and other hormones through diet, exercise and nutritional supplementation, that may lead to increased muscle mass and overall size,'' he said. ``That's where the movement is.''

San Diego conditioning coordinator Sam Gannelli said none of the Padres take Androstenedione, and he didn't believe steroids were widely used in baseball.

``Compared to every other sport, there's no time to heal in baseball,'' he said. ``In football, you have six days off after every game. In basketball, it's three or four days. These guys are going every day for six months. Steroids can really get you broken down. They can do a lot of harm in the long run.''