The octapeptide, angiotensin II (Ang II), the biologically active component of the renin-angiotensin system, elicits its multiple actions through the stimulation of specific surface receptors on various target organs. Although the existence of Ang II receptor subtypes has been suspected for some time, definitive evidence for Ang II receptor heterogeneity has been obtained only with the recently introduced nonpeptide Ang II receptor antagonists, exemplified by the prototypic compounds DuP 753 and PD 123177. The sites having high affinity for DuP 753 are designated as site 1 (AT1 receptors) and those having a high affinity for PD 123177 as site 2 (AT2 receptors). Unlike Ang I, Ang II, Ang III, and peptide antagonists, such as saralasin, which all are relatively nonselective ligands for both Ang II receptors, the peptides CGP42112A and p-aminophenylalanine6-Ang II show a marked preference for the AT2 site. The occurrence of the AT1 and AT2 receptor subtypes/binding sites identified so far appears widespread. The presence and proportion of these receptors vary significantly among different tissues/organs of the same species and within the same tissue/organ of different species. Despite the abundance of the AT2 site, its functional correlates remain to be determined. The DuP 753-sensitive site (AT1 receptor) mediates all the major Ang II-induced biological effects, including adrenal aldosterone and catecholamine secretion, release of catecholamines from sympathetic ganglia, central nervous system responses, and vasoconstriction.