The effects of naloxone on circulation and respiration were observed in 20 surgical patients during general anesthesia. Respiratory and circulatory changes were measured up to 15 minutes after naloxone injection at 5 minute intervals. Blood pressure and heart rate were slightly increased after naloxone injection during l5 minutes, but were not significantly increased. Arterial carbon dioxide partial pressure and pH were considerably decreased after naloxone injection during 15 minutes. Respiratory rate, tidal volume, and minute volume were markedly increased after naloxne injection during l5 minutes. In conclusion naloxone doses of 0.01 mg/kg seemed not to significantly change circulation and respiration during general anesthesia and naloxone slightly increased circulatory and respiratory function.