1 beta-Endorphin (2 micrograms injected into the lateral ventricles) produced a significant decrease in the urine outflow and in the excretion of Na+ and K+ in Brattleboro rats, animals suffering from severe diabetes insipidus. Morphine intracerebrally also produced antidiuresis, as compared to saline-treated controls. 2 Morphine injected intraperitoneally caused a dose-dependent decrease in the urine outflow, and in the excretion of Na+ and K+. 3. Rats chronically treated with morphine (72 h of morphine pellet implantation) were less sensitive to the antidiuretic effect of a challenge dose of morphine than control Brattleboro rats implanted with placebo pellets, but otherwise treated similarly. 4 After chronic morphine administration, Brattleboro rats became dependent on morphine. Challenge with 1 mg/kg naloxone (s.c.) precipitated an abrupt opiate withdrawal syndrome characterized, among other symptoms, by increased urination in contrast to the antidiuresis observed before naloxone.