The pituitary of the cyprinodont fish Poecilia secretes a hormone that is essential for tolerance of freshwater. Intact fish are euryhaline, but when hypophysectomized cannot tolerate fresh water, though living indefinitely in sea water or dilute sea water. In a typical experiment, ten hypophysectomized P. latipinna, transferred from dilute sea water (12 parts/1000) to fresh water at 25°, displayed severe distress symptoms within 8–46 hr. (mean, 22 hr.). When distressed in fresh water, hypophysectomized fish die in a few hours, but recover if returned immediately to dilute sea water (Ball, unpublished). The pituitary factor essential for freshwater life is probably not a neurohypophysial hormone, since it is secreted by ectopic pituitary transplants in P. formosa (Ball, Olivereau, Slicher & Kallman, 1965), in which the neurohypophysis is involuted and virtually devoid of histologically detectable neurosecretion (Olivereau & Ball, unpublished observations). Histological studies of the adenohypophysis of P. latipinna and of the