Isotocin is a nonapeptide of the arginine vasopressin-oxytocin family produced exclusively in the preoptic area (POA) of teleosts. As a homologue of mammalian oxytocin, studying fish nonapeptides has shown that Isotocin mediates social and reproductive behaviour in fishes. Functional isotocin is stored in granules at the axon terminal with a carrier peptide, neurophysin. Stimuli leads to isotocin dissociation, to be released into the bloodstream or to cross over into the brain. It binds to G-protein coupled receptors (GPCRs), which subtype isotocin binds to defines the following physiological action. It, like other nonapeptides, is a neuromodulator in the CNS, but when distributed by the bloodstream, it acts as a peripheral hormone, such as regulating osmoregulation. Isotocin levels are sex-dependent and linked to the reproductive cycle.

HPLC is a sensitive method to detect bioavailable isotocin and other nonapeptides. Measurement of mRNA levels of isotocin has been important to demonstrate the cyclical changes to regulate the endocrine calendar and diurnal rhythm. The level of isotocin is also considered a biomarker for aggression in behavioural indicators of fish welfare studies.