Arginine vasopressin (AVP) is a cyclic nonapeptide that acts by binding to a family of vasopressin receptors that includes V1a, V1b, and V2 receptors. In particular, V2 receptors are expressed in kidney where vasopressin exerts its antidiuretic action. V1a and V1b couple to Gq and calcium release, whereas V2 couples to Gs. Mutations in V2 result in X-linked nephrogenic diabetes insipidus, a syndrome in which the kidney is unable to concentrate urine, leading to dehydration and hypernatremia. Conversely, elevated levels of AVP lead to hyponatremia in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), congestive heart failure or cirrhosis, and V2 selective antagonists have been developed to treat these conditions.