Vasotocin is an oligopeptide homologous to oxytocin and vasopressin found in all non-mammalian vertebrates (including birds, fishes, and amphibians) and possibly in mammals during the fetal stage of development. Arginine vasotocin (AVT), a hormone produced by neurosecretory cells within the posterior pituitary gland (neurohypophysis) of the brain, is a major endocrine regulator of water balance and osmotic homoeostasis and is involved in social and sexual behavior in non-mammalian vertebrates. In mammals, it appears to have biological properties similar to those of oxytocin (stimulating reproductive tract contraction as in egg laying or birth) and vasopressin (diuretic and antidiuretic effects). It has been found to have effects on the regulation of REM sleep. Evidence for the existence of endogenous vasotocin in mammals is limited and no mammalian gene encoding vasotocin has been confirmed. AVT (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2), which occurs in the lamprey, represents the ancestral form in the phylogeny of the vertebrate neurohypophysial hormones. Gene duplication and point mutation have produced two distinct lineages, one involved in reproduction (oxytocin-like peptides) and the other in osmoregulation (vasopressin-like peptides). These hormones have remained highly conserved throughout evolution. Each is a peptide of nine amino acids derived from a preprohormone precursor by proteolytic cleavage, with an intramolecular disulfide bridge between the cysteine (Cys) residues; the C-terminal glycine (Gly) residue is amidated. Six of the residues have been found to be invariant in homologous peptides from numerous species of vertebrates. The vasopressin-like peptides, which differ in positions 3 and/or 8, include AVT and the mammalian hormones arginine vasopressin (Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, with isoleucine-3 of AVT changed to phenylalanine) and lysine vasopressin (isoleucine-3 changed to phenylalanine and arginine-8 changed to lysine). The oxytocin-like peptides, which differ in positions 4 and/or 8, include oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, with arginine-8 of AVT changed to leucine), mesotocin (arginine-8 changed to isoleucine), and isotocin (glutamine-4 changed to serine and arginine-8 changed to isoleucine); they differ from the vasopressin-like peptides in having a neutral amino acid in place of a basic amino acid at position 8. Oxytocin occurs in placental mammals; mesotocin occurs in amphibians, reptiles, and birds, and isotocin occurs in fishes. Biosynthesis AVT is synthesized as a preprohormone that includes a second peptide, neurophysin VT (neurophysins are carrier proteins that are secreted along with their passenger hormones); intracellular proteolytic processing generates the mature peptides. In the chicken (Gallus gallus), the 161-amino acid vasotocin-neurophysin VT preprohormone is encoded by the gene AVP, which is considered homologous to