Testosterone, oddly enough, is a chemical closely related to cholesterol. It was first isolated by a Dutch scientist in 1935 from mice testicles and successfully synthesized by the German biologist Adolf Butenandt. Although testosterone is often thought of as the definition of maleness, both men and women produce it. Men produce it in their testicles; women produce it in their ovaries and adrenal glands. The male body converts some testosterone to estradiol, a female hormone, and the female body has receptors for testosterone, just as the male body does. That's why women who want to change their sex are injected with testosterone and develop male characteristics, like deeper voices, facial hair and even baldness. The central biological difference between adult men and women, then, is not that men have testosterone and women don't. It's that men produce much, much more of it than women do. An average woman has 40 to 60 nanograms of testosterone in a deciliter of blood plasma. An average man has 300 to 1,000 nanograms per deciliter.

Testosterone's effects start early -- really early. At conception, every embryo is female and unless hormonally altered will remain so. You need testosterone to turn a fetus with a Y chromosome into a real boy, to masculinize his brain and body. Men experience a flood of testosterone twice in their lives: in the womb about six weeks after conception and at puberty. The first fetal burst primes the brain and the body, endowing male fetuses with the instinctual knowledge of how to respond to later testosterone surges. The second, more familiar adolescent rush -- squeaky voices, facial hair and all -- completes the process. Without testosterone, humans would always revert to the default sex, which is female. The Book of Genesis is therefore exactly wrong. It isn't women who are made out of men. It is men who are made out of women. Testosterone, to stretch the metaphor, is Eve's rib.

The effect of testosterone is systemic. It engenders both the brain and the body. Apart from the obvious genital distinction, other differences between men's and women's bodies reflect this: body hair, the ratio of muscle to fat, upper-body strength and so on. But testosterone leads to behavioral differences as well. Since it is unethical to experiment with human embryos by altering hormonal balances, much of the evidence for this idea is based on research conducted on animals. A Stanford research group, for example, as reported in Deborah Blum's book ''Sex on the Brain,'' injected newborn female rats with testosterone. Not only did the female rats develop penises from their clitorises, but they also appeared fully aware of how to use them, trying to have sex with other females with merry abandon. Male rats who had their testosterone blocked after birth, on the other hand, saw their penises wither or disappear entirely and presented themselves to the female rats in a passive, receptive way. Other scientists, theorizing that it was testosterone that enabled male zebra finches to sing, injected mute female finches with testosterone. Sure enough, the females sang. Species in which the female is typically more aggressive, like hyenas in female-run clans, show higher levels of testosterone among the females than among the males. Female sea snipes, which impregnate the males, and leave them to stay home and rear the young, have higher testosterone levels than their mates. Typical ''male'' behavior, in other words, corresponds to testosterone levels, whether exhibited by chromosomal males or females.

Does this apply to humans? The evidence certainly suggests that it does, though much of the ''proof'' is inferred from accidents. Pregnant women who were injected with progesterone (chemically similar to testosterone) in the 1950's to avoid miscarriage had daughters who later reported markedly tomboyish childhoods. Ditto girls born with a disorder that causes their adrenal glands to produce a hormone like testosterone rather than the more common cortisol. The moving story, chronicled in John Colapinto's book ''As Nature Made Him,'' of David Reimer, who as an infant was surgically altered after a botched circumcision to become a girl, suggests how long-lasting the effect of fetal testosterone can be. Despite a ruthless attempt to socialize David as a girl, and to give him the correct hormonal treatment to develop as one, his behavioral and psychological makeup was still ineradicably male. Eventually, with the help of more testosterone, he became a full man again. Female-to-male transsexuals report a similar transformation when injected with testosterone. One, Susan/Drew Seidman, described her experience in The Village Voice last November. ''My sex-drive went through the roof,'' Seidman recalled. ''I felt like I had to have sex once a day or I would die. . . . I was into porn as a girl, but now I'm really into porn.'' For Seidman, becoming a man was not merely physical. Thanks to testosterone, it was also psychological. ''I'm not sure I can tell you what makes a man a man,'' Seidman averred. ''But I know it's not a penis.''

The behavioral traits associated with testosterone are largely the cliche-ridden ones you might expect. The Big T correlates with energy, self-confidence, competitiveness, tenacity, strength and sexual drive. When you talk to men in testosterone therapy, several themes recur. ''People talk about extremes,'' one man in his late 30's told me. ''But that's not what testosterone does for me. It makes me think more clearly. It makes me think more positively. It's my Saint Johnswort.'' A man in his 20's said: ''Usually, I cycle up the hill to my apartment in 12th gear. In the days after my shot, I ride it easily in 16th.'' A 40-year-old executive who took testosterone for bodybuilding purposes told me: ''I walk into a business meeting now and I just exude self-confidence. I know there are lots of other reasons for this, but my company has just exploded since my treatment. I'm on a roll. I feel capable of almost anything.''

When you hear comments like these, it's no big surprise that strutting peacocks with their extravagant tails and bright colors are supercharged with testosterone and that mousy little male sparrows aren't. ''It turned my life around,'' another man said. ''I felt stronger -- and not just in a physical sense. It was a deep sense of being strong, almost spiritually strong.'' Testosterone's antidepressive power is only marginally understood. It doesn't act in the precise way other antidepressants do, and it probably helps alleviate gloominess primarily by propelling people into greater activity and restlessness, giving them less time to think and reflect. (This may be one reason women tend to suffer more from depression than men.) Like other drugs, T can also lose potency if overused. Men who inject excessive amounts may see their own production collapse and experience shrinkage of their testicles and liver damage.

Individual effects obviously vary, and a person's internal makeup is affected by countless other factors -- physical, psychological and external. But in this complex human engine, testosterone is gasoline. It revs you up. A 1997 study took testosterone samples from 125 men and 128 women and selected the 12 with the lowest levels of testosterone and the 15 with the highest. They gave them beepers, asked them to keep diaries and paged them 20 times over a four-day period to check on their actions, feelings, thoughts and whereabouts. The differences were striking. High-testosterone people ''experienced more arousal and tension than those low in testosterone,'' according to the study. ''They spent more time thinking, especially about concrete problems in the immediate present. They wanted to get things done and felt frustrated when they could not. They mentioned friends more than family or lovers.''