This study was aimed to determine 1) the changes of the anesthetic and analgesic actions of inhalation of subanesthetic concentration of nitrous oxide, and 2) the possible antagonism by naloxone of these actions. Auditory response time (anesthetic action) and pain threshold (analgesic action) were examined before, during and after inhalation of subanesthetic concentration of nitrous oxide in six normal volunteers who were treated with or without intravenous administration of naloxone 1.4 mg. Analgesic action remained at 30 min after nitrous oxide inhalation, while auditory response time returned to control values. Both anesthetic and analgesic effects were unchanged during 100 min inhalation or administration of naloxone. These results indicate that the acute tolerance to these actions of nitrous oxide will not occur, and that naloxone will not show a significant antagonistic action as is observed with morphine.