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The effect of naloxone, a narcotic antagonist, on the response of animals to painful stimuli during anesthesia was studied. Bats were anesthetized with cyclopropane, halothune, or enflurane in groups of 12. Following induction, inspired anesthetic concentration was gradually reduced to a point at which 35–60 per cent of animals responded to tail clamping. Thereafter the anesthetic concentration was held constant for 30 minutes. Bats in each group then received saline solution or naloxone, 10 mg/kg, given intravenously. The response to tail clamping was retested 5 minutes later. In additional experiments EEG's were recorded from rats anesthetized with one of these anesthetics. After a stable light plane of anesthesia had been attained, each animal was given naloxone, 10 mg/kg, iv, and the EEG recorded for an additional 5 minutes. In the tail-clomping experiments, naloxone approximately doubled the number of rats responding during cyclopropane, halothane, or enflurane anesthesia. The EEC patterns of several animals anesthetized with either cyclopropane or halothane changed to patterns consistent with lighter planes of anesthesia after naloxone administration. That naloxone alters the depth of inhalational anesthesia suggests that anesthetics may release an endogenous morphine-like factor (MLF) in the central nervous system.