I. Autoradiographic techniques coupled with computerized microdensitometry and comparison with 125[ standards were used to characterize and quantitate receptors for neuropeptides in rat brain and adrenal and pituitary glands. 2. These techniques are rapidly performed, anatomically precise, and more sensitive than membrane binding techniques. They permit the determination of complete saturation curves and Scatchard analysis in discrete nuclei of the rat brain and in single rat pituitary and adrenal glands. 3. Angiotensin II (AII) receptors were quantitated after incubation of 16-pm tissue sections with the AII agonist l ZSI-[Sarl]-AII. 4. High-affinity, high-density AII receptors were present in the organon subfornicalis, organon vasculosum laminae terminalis and nuclei triangularis septalis, suprachiasmatis, and paraventricularis of the rat and in rat adrenal capsule-zona glomerulosa area, adrenal medulla, and anterior pituitary. 5. These techniques could be used for precise localization and quantitation of other neuropeptide receptors in single rat brain nuclei, after optimizing the assay conditions and provided that suitable 125I ligands are available.