Radioactivity was analyzed in tissue homogenates and purified cell nuclear fractions in the pituitary and 9 brain regions 2 h after an iv [7-3H]testosterone infusion into gonadectomized-adrenalectomized adult male and female rats. Unchanged testosterone (T) and its metabolites, 5alpha-dihydrotestosterone (DHT) and estradiol-17beta (E2), were the predominant steroids recovered from cell nuclear fractions in all brain regions examined. Highest levels of E2 as a T metabolite were found in nuclear fractions from the amygdala, followed by the hypothalamus, preoptic area and septum, while levels of DHT as a T metabolite were highest in nuclear fractions from the pituitary, followed by the hypothalamus and the septum. The regional pattern was similar in both sexes. Low levels of DHT and E2 recovered from serum indicated these metabolites were probably formed in situ. Regional distributions of cell nuclear retained DHT and E2 as T metabolites were compared with the respective regional distributions observed after either [1,2-3H]DHT or [6,7-3H]E2 infusion. Cell nuclear levels of E2 as a T metabolite did not correlate well with nuclear retained radioactivity after [3H]E2 injection, but cell nuclear levels of DHT as a T metabolite did correlate with nuclear radioactivity after [3H]-DHT infusion. No apparent sex differences were observed in nuclear-associated radioactivity after [3H]-E2 or [3H]DHT injections. The results emphasize the potential importance of local aromatization in the brains of rats of both sexes, suggest the existence of a DHT receptor site at the brain cell nuclear level, and confirm the existence of estradiol-17beta cell nuclear receptors in the adult male rat brain.