The actions and side effcts of the benzodiazepine antagonist flumazenil were evaluated in a clinical study in which midazolam was used as an anesthetir induction agent. Anesthesia was maintained with nitrous oxide-oxygen-fentanyl. Sixty-five adult surgical patients received either 0.3 mg (3 ml) of flumazenil (33 cases) or 3 ml of saline (32 cases) at the conclusion of surgery. Blood pressure, pulse rate, and respiratory rate were checked before premedication, just before, 5 minutes, 30 minutes, and 120 minutes after the administration flumazenil or saline. Also level of consciouseness, orientation in time and space, and cooperativeness along with end tidal carbon dioxide and hemoglobin oxygen saturation were evaluated. Various larboratory parameters were checked before premedication and 24 hours after operation. The results were as follows: 1) The flumazenil group revealed improved level of consciousness, orientation in time and space, and cooperativeness which began just after the administration, and was maintained 30 minutes thereafter. 2) There was no difference in the changes of laboratory parameters between the two groups. 3) The changes of blood pressure, pulse rate, respiratory rate, and SaO2 showed no difference between those two groups. 4) The end tidal concentrations of carbon dioxide after drug administration were significantly lower in the flumazenil group than those of the saline group. 5) Tolerance to flumazenil (0.3 mg) was good without any side effect.