Vasopressins (VPs) are mammalian VP family nonapeptides. VPs are composed of an intramolecular ring structure flanked by two cysteine residues and a C-terminal extension with a basic amino acid residue at position 8. VP causes antidiuresis, blood pressure elevation, stress response, metabolic actions, and various behaviors through three VP receptors. A natural knockout model of AVP, the Brattleboro rat, shows the characteristic polyuria and polydypsia of central diabetes insipidus (DI). On the other hand, loss-of-function mutation of V2aR or aquaporin-2 causes congenital nephrogenic DI. Knockout animals of V1aR and V1bR showed hypotension, renal tubular acidosis, and many other defects, including those in social recognition, and reduced anxiety-like or aggressive behavior.