The effects on the duration of sleeping time (ST) of the opiate receptor antagonists, naloxone and naltrexone, were determined in rats anesthetized by intraperitoneal injection of ketamine, halothane, or pentobarbital. Intracerebroventricular administration of naloxone shortened the duration of sleeping time induced by all three anesthetic agents in a dose-related manner. Centrally administered naltrexone (240 microgram) and systemically administered naloxone (50 mg/kg) prolonged the duration of pentobarbital sleeping time without altering duration of ketamine or halothane sleeping time. Naltrexone (120 microgram) had no effect on the duration of ST. This study does not support a role for opiate receptor regulation of the duration of sleeping time. The evidence supports the hypothesis that naloxone may govern the duration of narcosis through the activation of an opposing arousal system in the CNS, unrelated to pharmacologic competition for opiate receptors.