The purpose of this study was to generate an antibody against the binding site of the vasopressin receptor. Recently it has been demonstrated that complementary RNA sequences may encode interacting peptides. We determined whether or not an antibody directed against a peptide (PVA) specified by RNA complementary to the mRNA of rat arginine vasopressin (AVP) would recognize the AVP receptor. The antibody was purified sequentially over a protein A and then a PVA affinity column. A specific anti-PVA antibody affinity column also was made. The specific anti-PVA antibody recognized two proteins with molecular weights of 76 and 70 kD in homogenates from kidney and brain tissue containing the hypothalamus/thalamus/septum. This antibody also blocked binding of <sup>3</sup>H-AVP to primary cultured neuronal cells, whereas the nonspecific antibody did not. Furthermore, the blocking efficiency of the antibody increased when more of the specific anti-PVA antibody was present. We also determined whether or not the antiserum contained any biological activity by observing urine volume and urine osmolality in the presence or absence of preimmune or immune serum. Only the immune serum was able to reverse the antidiuretic and urine-concentrating effects of AVP, suggesting that the antibody antagonized the effects of AVP on the kidney. Taken together with the observation that this antibody did not bind to vasopressin, these data strongly indicate that an antibody to the vasopressin receptor binding site has been made and add further support to the molecular recognition theory.