1 The specificity of oestrogen receptor in rat brain regions (hypothalamus, amygdala and cortex), pituitary and uterus was studied by measurement of the inhibition of 17beta-oestradiol high affinity binding in cytosol, in the presence of unlabelled putative inhibitors of binding. 2 Binding was inhibited only by those of the compounds tested that possessed oestrogen agonist or antagonist activity. The affinities were estimated and the ranking order of the compounds was the same in all tissue sources of cytosol and corresponded to the ranking order of agonist or antagonist potency. 3 There were some significant differences between some of the estimated affinities in different tissues, these being seen most commonly between pituitary and hypothalamus on one hand and uterus on the other. 4 The possibility of heterogeneity of oestrogen receptor is discussed.