Plasma sodium concentrations in freshwater Poecilia latipinna fall by about 25% within 24 hours of removal of the pituitary gland. A single injection of ovine prolactin (0.1 to 1.4 IU per gram weight) 4 hours after hypophysectomy partially or completely prevents this fall in sodium levels, the response being dose-dependent. ACTH, growth hormone, TSH, alpha-MSH, oxytocin, vasopressin, arginine vasotocin and isotocin (each tested at a low and a high dose) were all unable to influence plasma sodium levels in this protocol. In a slightly hypotonic dilute seawater, chronic prolactin therapy maintained normal plasma sodium levels in hypophysectomized fish, but ACTH, growth hormone, TSH and alpha-MSH were ineffective. The gonadotrophins can be excluded from participation in the maintenance of plasma sodium in freshwater Poecilia , since natural or experimental alterations in the pituitary-ovary axis have no effect on the ability of the fish to survive in freshwater. Thus the sodium-maintaining property of prolactin is not shared by the other pituitary hormones, and this may be taken to indicate a specific physiological function of the teleostean prolactin-like hormone. Plasma concentrations of potassium and calcium did not alter in parallel with sodium, and were not affected by hypophysectomy.