The coding region of the human vasopressin type 2 receptor gene bears mutations in the individuals affected with congenital nephrogenic diabetes insipidus, a disease characterized by the inability of the kidney to concentrate urine in response to vasopressin. Although it is assumed that the mutations result in loss of receptor function, proof of this hypothesis is lacking. We introduced one of these naturally occurring point mutations leading to a single amino acid change (Arg137–>His) into wild type cDNA. The mutant protein was expressed, and the functional properties of the receptor were examined. The mutant receptor exhibited an unaltered binding affinity for vasopressin compared to the wild type but failed to stimulate the Gs/adenylyl cyclase system. These data provide biochemical proof that the mutant receptor is the cause of the disease.