The nuclear uptake and retention of [3H]testosterone or one of its metabolites and the aromatization of testosterone to estrogen were examined in the Swiss--Webster mouse. Castrated male mice were injected with 0.2 micrograms of either [1 alpha, 2 alpha-3H(N)]testosterone or [1 beta, 2 beta-3H(N)]testosterone per 100 g of body weight and killed one and one-half hours later. The brains were removed and processed for autoradiography. A nuclear localization of testosterone or one of its metabolites was found in the nucleus (n) interstitialis striae terminalis, n. preopticus medialis, n. premamillaris ventralis and n. amygdaloideus medialis in animals injected with [1 alpha, 2 alpha-3H(N)]testosterone. In animals injected with [1 beta, 2 beta-3H(N)]testosterone a nuclear localization was found in only n. interstitialis striae terminalis, n. premamillaris ventralis and n. amygdaloideus medialis. The results suggest testosterone is aromatized to estrogen in n. preopticus medialis ventralis in the Swiss--Webster mouse. Together with previous data, these data suggest (1) the uptake and retention of testosterone or one of its androgenic metabolites and the aromatization of testosterone to estrogen varies between strains of mice and (2) there are two separate uptake and retention systems (receptors?) for testosterone and dihydrotestosterone in the brain in all animals studied thus far with autoradiographic techniques.