Plasma concentrations of prolactin were measured in male sheep (wethers, n = 8) during 120 min exposure to mild physical stress (restraint), and also following I.V. injection of 30 micrograms ovine corticotrophin-releasing factor (CRF), when the animals were water replete and when they had been deprived of water for 48 h. Restraint stress produced a small increase in prolactin secretion (n.s.) when the animals were euhydrated and a large increase (P less than 0.001) when they were dehydrated. Administration of CRF had no effect on prolactin concentrations in either experimental condition. In a further study in which euhydrated animals (n = 6) were subjected to restraint while receiving infusions of vasopressin (1 microgram/h), there was no enhancement of basal or stress-induced prolactin secretion. Thus, dehydration enhances stress-induced prolactin release in sheep through a process that does not involve a pituitary action of CRF or circulating vasopressin.