BACKGROUND: Respiratory depression with high dose of propofol during induction is one of the major complications. We studied the effects of midazolam as premedicant on frequency and duration of apnea and frequency of loss of consciousness in relation to single dose of propofol. METHODS: We selected 194 adult patients who had clear consciousness and no depression of respiration. We allocated patients randomly to control group and midazolam group. In midazolam group, we injected 0.06mg/kg of midazolam intravenously 10min before induction, and in control group, we did nothing. Under mask oxygenation with 100% oxygen, we administered a bolus of propofol (1, 1.5, 2 mg/kg to subgroup 1, 2, 3 respectively) intravenously. The change of respiration and loss of consciousness were observed. RESULTS: The frequency and duration of apnea increased with the dose of propofol in both control and midazolam group. But there were no difference between groups except frequency of apnea with 1.5 mg/kg of propofol. In control group, frequency of loss of consciousness increased with the increasing dose of propofol. But in midazolam group, nearly all the patients was slept without difference by the dose. CONCLUSIONS: Premedication with midazolam reduce the sleeping dose of propofol to induce anesthesia, so the frequency and duration of apnea which is caused by high dose of propofol can be decreased.