Arginine vasopressin (AVP) acts on at least two receptor types, classified on the basis of their second messengers. The V1receptor acts via mobilization of intracellular calcium through phosphatidylinositol hydrolysis and influences blood pressure and hepatic glycogenolysis. The V2receptor acts via cAMP through activation of adenylate cyclase and causes anti-diuresis. Previous studies of the different AVP receptors have been hampered by the use of nonselective radioligands, such as [3H]AVP (which binds to all types of V1and V2receptors, certain oxytocin receptors, and neurophysins) as well as the difficulty of measurement of second messengers. This paper describes the use of selective V1and V2radioligands with in vitro autoradiography to study V1and V2binding sites in rat tissues. [125I][l-(β-mercapto-(8,β-cyclopentamethylene propionic acid), 7-sarcosine] arginine vasopressin ([125I][d(CH2)5, Sarcosine7]AVP), a selective V1antagonist radioligand, bound to regions of the brain, testis, superior cervical ganglion, liver, blood vessels, and renal medulla. Pharmacological characterization of [125I] [d(CH2)5, Sarcosine7]AVP binding was consistent with that expected for binding to V1receptors. There was no specific binding demonstrable to pituitary, renal glomeruli, gut, heart, spinal cord, ovary, adrenal medulla, or adrenal cortex. [3H]l-deamino [8-D-arginine] vasopressin ([3H]DDAVP), a potent V2receptor agonist radioligand, was used to study V2receptors. Specific binding was only identified in the kidney consistent with the known distribution of antidiuretic V2receptors on renal collecting tubules. No binding was demonstrated on endothelium or liver where DDAVP might influence clotting factor release, nor in the brain, spinal cord, sympathetic ganglia, heart or vascular smooth muscle, regions where DDAVP might cause vasodilatation.These studies demonstrate the use of these radioligands to study V1and V2receptors in a variety of tissues. Also, since these ligands are selective they are of particular use to study the different receptor subtypes in tissues where V1and V2receptors coexist, such as in the kidney.