Circulating titers of testicular and gonadotrophic hormones differ widely among conspecific mammalain males. The behavioral and physiological consequences of these differences were examined by injecting gonadally intact male rats with extra testosterone or an antiandrogen. Serum testosterone values increased and decreased, respectively, relative to control animals, although all values remained within normal physiological ranges. Results suggested that hormone-sensitive behavior and physiology were related to suprathreshold androgen differences in a dose-response fashion among virgin males. The relationship was more complex among sexually experienced males. Additional testosterone to the experienced males increased the frequency of both aggressive and sexual responding as well as the activity of sex accessory glands. Reductions of circulating testosterone, however, had minimal effects on behavior despite substantial atrophy of hormone-sensitive tissues. Indeed, fecundity was independent of hormone therapy as all experienced males successfully fathered pups. Suprathreshold hormone differences in natural populations of males may reflect sexual selection that is maintained by female choice of a mate.