The 125I-labeled agonist analog [1-sarcosine]-angiotensin II ([Sar']AII) bound with high specificity and affinity (Ka = 2 x 109 M-1) to a single class of receptor sites in rat brain. This ligand was used to analyze the distribution of All receptors in rat brain by in vitro autoradiography followed by computerized densitometry and color coding. A very high density of All receptors was found in the subfornical organ, paraventricular and periventricular nuclei of the hypothalamus, nucleus of the tractus solitarius, and area postrema. A high concentration of receptors was found in the suprachiasmatic nucleus of the hypothalamus, lateral olfactory tracts, nuclei of the accessory and lateral olfactory tracts, triangular septal nucleus, subthalamic nucleus, locus coeruleus, and inferior olivary nuclei. Moderate receptor concentrations were found in the organum vasculosum of the lamina terminalis, median preoptic nucleus, medial habenular nucleus, lateral septum, ventroposterior thalamic nucleus, median eminence, medial geniculate nucleus, superior colliculus, subiculum, preand parasubiculum, and spinal trigeminal tract. Low concentrations of sites were seen in caudate-putamen, nucleus accumbens, amygdala, and gray matter of the spinal cord. These studies have demonstrated that All receptors are distributed in a highly characteristic anatomical pattern in the brain. The high concentrations of All receptors at numerous physiologically relevant sites are consistent with the emerging evidence for multiple roles of All as a neuropeptide in the central nervous system.