The identification of the different molecular causes of congenital nephrogenic diabetes insipidus (NDI), a disorder characterized by renal insensitivity to the antidiuretic effect of arginine vasopressin, has been of indispensable importance for understanding the cellular processes involved in diuresis and antidiuresis. In most cases, NDI is X-linked and caused by mutations in the vasopressin type-2 receptor (V2R) gene. Mutations in the aquaporin-2 (AQP2) water channel gene are responsible for the autosomal recessive and rare dominant forms of NDI. By in vitro expression, it has been shown that the majority of V2R mutants and all AQP2 mutants found in recessive NDI are misfolded and retained within the endoplasmic reticulum (ER). Functional analysis of one of the mutations identified in dominant NDI showed that this mutant is properly folded and transported out of the ER, but is retained in the Golgi region. In addition, this mutant, in contrast to mutants found in recessive NDI, is able to heterotetramerize with wild-type AQP2. The resulting complex is hindered in its transport to the membrane, a finding that explains the dominant-negative effect of this mutation. Several new methodologies focused on the molecular defects causing NDI are presently being investigated in vitro and might eventually develop into useful therapeutic strategies.