The effects of acupuncture analgesia were studied using the change in halothane MAC in volunteers. Halothane MAC under electrical acupuncture stimulation was reduced to 86.2 ± 11.1% from the control value. After naloxone administration the level of the MAG was raised to the control level and the increment was 19.l ± 14.8%. Naloxone itself did not change the halothane MAC in the same subjects. These results suggest that the changes in halothane MAC under acupuncture stimulation are caused by the release of an endogenous analgesic substance in the brain
