Abstract High affinity binding of 17 β-oestradiol was measured in cytosols of hypothalamus, amygdaloid region, pituitary and (in females) uterus of adult male and female rats. There were no differences between intact or gonadectomised male and female animals in any of the tissues in the equilibrium dissociation constants (K d). The number of available binding sites (n) in brain and pituitary in intact females at metoestrus is higher than at proestrus but only in hypothalamus is n greater than in ovariectomised animals. Binding sites in male hypothalamus, amygdala and pituitary are significantly less than in metoestrous females; the sex difference is seen also in gonadectomised rats but is significant only in pituitary. In all female animals the highest concentration of binding sites in the hypothalamus is in the anterior part and the lowest in the posterior part. The distribution of binding sites andK d values in adult females treated neonatally with testosterone propionate were not different from those of intact proestrous rats. In intact males the highest level of n was in mid-hypothalamus; after gonadectomy the pattern reverted to that in females. It is suggested that these results support the concept that testicular androgen is converted in brain to a substance with affinity for cytosol oestrogen receptor.