Previous studies have used new angiotensin II (AII) receptor subtype selective compounds to localize AII receptor subtypes within discrete rat brain nuclei. The purpose of this autoradiographic study was to extend these preliminary findings and provide a comprehensive analysis of AII binding sites in 22 rat brain nuclei and the anterior pituitary, to include estimates of the binding affinity for <sup>125</sup>I sar<sup>1</sup> ile<sup>8</sup> AII (<sup>125</sup>I SIAII) at each nucleus, and determine the fractional distribution of each subtype at each nucleus. Estimates of K<sub>D</sub> in separate experiments revealed that AT<sub>1</sub> nuclei had a consistently higher affinity for <sup>125</sup>I SIAII than AT<sub>2</sub> nuclei (0.66 vs. 2.55 n M). Displacement of subsaturating concentrations of <sup>125</sup>I SIAII by 10<sup>–8</sup>–10<sup>–4</sup> M DuP753 (selective for the AT<sub>1</sub> subtype) or PD123177 (selective for the AT<sub>2</sub> subtype) indicated that approximately half of the brain regions surveyed contained predominantly AT<sub>1</sub> sites and half contained predominantly AT<sub>2</sub> sites. Binding was partially displaced by both compounds in several regions and two site analyses were performed to estimate the distribution of subtypes within each nucleus. The data were then corrected for differential occupancy by <sup>125</sup>I SIAII. Brain nuclei associated with cardiovascular or dipsogenic actions of AII, e.g., subfornical organ, organum vasculosum of the lamina terminalis, median preoptic nucleus, nucleus of the solitary tract and area postrema, contained pure, or almost pure, populations of AT<sub>1</sub> receptors. The functions of AII in brain regions containing predominantly AT<sub>2</sub> binding sites, e.g., thalamus, colliculi, inferior olive and locus ceruleus, remain undefined. Thus, AII binding sites in the rat brain have been differentiated into two subtypes with similar characteristics to those reported in peripheral tissues. However, the unexpected finding that they can be differentiated on the basis of their affinity for <sup>125</sup>I SIAII raises questions concerning their coidentity with peripheral receptor subtypes.