Receptor-estrogen (R-E) complex in the nuclear fraction of the pituitary and hypothalamus of male and female immature rats was investigated. 3 anterior pituitaries or 3 hypothalami were incubated with tritiated estradiol alone or tritiated estradiol plus diethylstilbestrol (DES) and the amount of estradiol in the nuclear fraction was determined. No marked differences were observed in the dissociation constant of R-E complex when male and female pituitaries were compared. In the female hypothalamus, nuclear complexes were measurable and appeared to be similar to those of the pituitary with respect to dissociation constant; however, the R-E complex concentration was much lower. In the male hypothalamus no nuclear R-E complex was observed. It was concluded that normal circulating levels of testosterone have no effect on the estradiol receptor of the anterior pituitary but that either the level of receptor or the ability of the receptor to move from the cytoplasm to the nuclear fraction has been altered during the development of the male hypothalamus.