The effective dose, usefulness and side effects of flumazenil, a specific benzodiazepine antagonist, have been investigated in 72 sleeping patients after the end of the surgical operation who had received flunitrazepam 0.03 mg.kg-1, a long acting benzodiazepine. The patients received intravenous injections of flumazenil 0.1 mg, 0.2 mg, 0.4 mg and 0.8 mg per person as initial doses. Four minutes after the injection of flumazenil, the percentages of the patients who were awake, were 47.4% in 0.1 mg group, 82.4% in 0.2 mg group, 82.4% in 0.4 mg group and 88.2% in 0.8 mg group respectively. There were several side effects observed in all the groups except 0.2 mg group. However, they presented no clinical problems. Therefore, flumazenil 0.2 mg as the initial dose, is considered appropriate.