Doses of testosterone that fully activate male reproductive behavior in castrated adult male hamsters fail to elicit mounting and intromissions in prepubertal castrates, even when circulating levels of testosterone are equivalent in the two age groups. We hypothesize that this differential responsiveness to testosterone is mediated at least in part by the efficacy with which testosterone in the hypothalamus is aromatized to estradiol, an important hormone mediating male sexual behavior. Therefore, hypothalamic aromatase activity, as measured by the conversion of [3H]testosterone to [3H]estradiol in tissue homogenates, was assessed in four separate experiments: 1) intact prepubertal and adult male golden hamsters, 2 and 3) castrated adult or prepubertal males that received either a 0- or 2.5-mg dose of testosterone, and 4) castrated adult and prepubertal males treated with the 2.5-mg dose oftestosterone. These studies demonstrate that hypothalamic aromatase activity is significantly higher in adult males compared with prepubertal males, and that hypothalamic aromatase activity is increased by testosterone to the same extent in both the adult and prepubertal male hamster. Therefore, the failure of testosterone-treated castrated prepubertal male hamsters to engage in the full suite of male reproductive behaviors is not due to the inability of testosterone to be converted into estradiol in the hypothalamus. Differences in the ability of testosterone to increase aromatase activity in other brain regions, or differences in the action of testosterone and/or estradiol on other cellular processes must account for the inability of testosterone to facilitate male reproductive behavior in juvenile males.