The molecular cloning and characterization of receptors for [Arg8] vasopressin and oxytocin were recently accomplished. These receptors form a subfamily among the large number of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors with seven transmembrane domains. The molecular cloning of the human V2 receptor was rapidly followed by the identification of mutations in the V2 receptor gene segregating with the clinical phenotype in families with X-linked nephrogenic diabetes insipidus. These naturally occurring mutations will be useful to identify critical functional regions of the vasopressin V2 receptor. Carrier detection and early diagnosis of affected male infants are available and can avert the physical and mental retardation that are the consequences of episodes of dehydration. Together with the recent cloning of the vasopressin-regulated water channels in the apical membrane of the collecting tubule, these developments will enable direct investigation of the mammalian concentrating mechanism.