Loss-of-function mutations in the V2 vasopressin receptor (V2R) cause X-linked congenital nephrogenic diabetes insipidus (CNDI). These mutations are a model system for studying the structure-function relationship in G protein-coupled receptors. In order to decipher their molecular effects, six different missense mutants of the V2R gene from NDI patients were analyzed in transfected cells for residual signal transduction capability. Five (L53R, L81F, L83P, P95L, and W323R) showed no ability to elevate intracellular cAMP levels in response to physiologic or supra physiologic stimulation with arginine vasopressin (AVP). The lack of response could not be overcome by overexpressing the mutant receptors. In contrast, the sixth mutation (Y205C), provided maximal activity that was about 50% of wild-type when stimulated with 10-7M AVP (≈100 times physiologic). At 10-8M AVP, overexpression of the mutant receptor could overcome the difference in activity compared with wild-type. These results indicate that the Y205C mutation is able to respond to AVP stimulation, albeit with sub-normal efficiency, and raises the possibility that patients with this mutation may respond to high-dose dDAVP therapy.