Congenital nephrogenic diabetes insipidus (CNDI) is a rare inherited disorder characterized by renal tubular insensitivity to the antidiuretic effect of arginine vasopressin (AVP). In a large majority of the cases, nephrogenic diabetes insipidus is an X-linked recessive disorder caused by mutations in the AVP V2 receptor gene (AVPR2). In the remaining cases, the disease is autosomal recessive or dominant and, for these patients, mutations in the aquaporin 2 gene (AQP2) have been reported. Fourteen probands belonging to 12 families were analyzed by single-strand conformational polymorphism and direct sequencing of the AVPR2 and AQP2 genes. Ten mutations of the AVPR2 gene (six previously reported mutations and four novel mutations: G107E, W193X, L43P, and 15delC) were identified. Three mutations of the AQP2 gene were also identified in two patients: the first patient is homozygous for the R85X mutation and the second is a compound heterozygote for V168 M and S216P mutations. Extrarenal responses to infusion of the strong V2 agonist 1-desamino-8-D-arginine vasopressin allowed AVPR2- and AQP2-associated forms of CNDI to be distinguished in three patients. This test also identified an unexpectedly high urinary osmolality (614 mosmol/kg) in a patient with a P322S mutation of AVPR2 gene and a mild form of CNDI.