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The effects of two narcotic antagonists, naloxone and naltrexone, on minimum alveolar anesthetic concentration (MAC) were studied during halothane anesthesia in the dog. Following induction of anesthesia, MAC was determined in duplicate. Each dog was then given one of the two narcotic antagonists and MAC was redetermined in duplicate: five dogs were given naloxone, 3 mg/kg IV, followed by an intravenous infusion of naloxone, 0.025 mg/kg/min; five dogs received naltrexone, 5 mg/kg IV. MAC was unaffected by the narcotic antagonists. Failure of narcotic antagonists to reverse halothane anesthesia suggests that the endogenous opioid peptides are not involved in the anesthetic state produced by halothane.