Pituitary arginine vasotocin (AVT) secretion is sensitive to the osmotic challenge associated with transfer of euryhaline teleosts between sea water (SW) and fresh water (FW). Pituitary AVT content in FW-adapted flounders greatly exceeds that in SW-adapted fish. Plasma AVT concentrations are in the range 10(-12)-10(-11) M (1-100 pg/ml). In euryhaline species, like the eel, flounder and trout, there were no consistent, marked differences in plasma AVT concentrations between FW- and SW-adapted fish. In SW- but not FW-adapted flounders plasma AVT and sodium concentrations are correlated. During the initial period of acclimation from FW to SW eels, show a transitory rise in plasma AVT concentration which is associated with a transitory increase in plasma Angiotensin II. In view of the range of plasma AVT concentration observed in SW- and FW-adapted fish, it is evident that of the described dose-dependent effects of AVT on urine production, only the antidiuretic responses are likely to be of physiological significance. In addition to the presence of a V1-type vascular receptor for AVT, the nephron also possesses a V2-type receptor, coupled to adenylate cyclase. In the gill tissue AVT receptors are also present, but in this tissue receptor occupancy leads to inhibition of cAMP production rather than the stimulation observed in renal tissue. The functional significance of the gill AVT receptor remains to be established.