Glucocorticoids bind with high affinity to intracellular receptors located in high density within discrete regions of the rodent and primate brain. The binding of [3H]corticosterone was compared in the brains of male vs female rats. The number and affinity of cytosol receptors in the hippocampus and hypothalamus were examined in vitro. The cytosolic binding capacity of the hippocampus is greater in the female than in the male. This difference in binding capacity is not dependent on the presence of gonadal steroids: the effect of gonadectomy was not significant for either sex. The difference is not due to transcortin since the binding capacity of [3H]dexamethasone is also greater in the female hippocampus. Receptor affinity in the female hippocampus is half that of the male value. In the hypothalamus, the dimorphism is in the opposite direction: the number of [3H]corticosterone cytosolic binding sites was found to be greater in the male. The male hypothalamus also showed a greater affinity for [3H]corticosterone than did the female. Ovariectomy increased the number of binding sites in the female hypothalamus. In vivo nuclear uptake of a tracer dose of [3H]corticosterone was determined in animals having intact gonads. The percent of tissue [3H]corticosterone present in cell nuclei from 4 brain regions, including the hippocampus and hypothalamus, was calculated per unit DNA. The concentrations of [3H]corticosterone in nuclei relative to tissue homogenates were higher in females than males for the 4 brain regions, but not for the pituitary or liver. The data are interpreted as suggesting that glucocorticoid secretion under basal conditions and during stress may differentially effect specific brain structures in male vs female rats.