The metabolism of 4-[4-14C]androstene-3,17-dione in the microsomal fraction of livers from male and female rats was investigated after hypothalamic deafferentation at two levels. It was found that frontal deafferentation at the retrochiasmatic level caused a complete 'feminization' of hepatic steroid metabolism in the male rat but was without effect in the female animal. Transection rostral to the suprachiasmatic nuclei was without effect in both sexes. A complete transition from male to female hepatic steroid metabolism after retrochiasmatic deafferentation was reached on day 4 after the operation and persisted for at least 10 weeks. The present results, taken together with previous investigations, indicate that the release of a 'feminizing' factor from the pituitary gland of the male rat is inhibited by a factor produced in, or transported through, the periventricular anterior hypothalamic region including the suprachiasmatic area. No effect on the hepatic steroid metabolism was observed after binding of the rats suggesting that a diurnal rhythm is not essential to this control mechanism.