Vasopressin produced analgesia in mice as estimated by using abdominal constriction tests (ED50 8.5 micrograms/kg i.v.) or hot plate method (ED50 63 micrograms/kg i.v.). However, vasopressin (10 micrograms/kg i.v.) produced no depression of locomotor activity in mice. Vasotocin had slight analgesic action; oxytocin or norepinephrine had none and there was no direct correlation between pressor response and analgesia. The analgesic action was nonopiate in nature as it was uninfluenced by the narcotic antagonist naltrexone at 5 to 15 mg/kg, but it was reserved by a vasopressin antagonist. Intraventricular administration of vasopressin (1-10 micrograms/kg) to mice produced no significant analgesia, suggesting a primarily peripheral locus of analgesic action. Vasopressin may play a role as an endogeneous pain regulating substance.