BACKGROUND: Previous reports have demonstrated the synergistic interaction of midazolam and propofol in the induction of hypnosis. But there haer been some different views expnrsscd as to whether the synergism extended to hemodynamic effects. So we studied the effect of the co-administration of midazolam on induction dose, hemodynamic response, and recovery with the use of continuous infusion of propofol for induction, and the maintenance of anesthesia. METHODS: Thirty-five patients undergoing elective surgery within 2 hours were randomly assigned to one of two groups formed according to the induction agents: Group P (continuous propofol infusion 1,200 mg/h), Group MP (midazolam 2 mg followed by continuous propofol infusion 1,200 mg/h). After induction, anesthesia was maintained with fentanyl (50 microgram), N2O (70%), andpropofol (5 15 mg/kg/h). Outcome measures were propofol doses (induction and maintenance), hemodynamic responses (heart rate, blood pressure) during the induction period, emergence time (eye-opening to command), postoperative nausea and dizziness. RESULTS: The induction dose of propofol was 29% less in Group MP compared to Group P but there was no significant difference in maintenance doses between the two groups. Heart rates showed no differences between the two groups, but the changes of mean arterial pressures from base line at 30 sec, 2 min and 5 min after intubation were greater and the emergence time was delayed in Group MP compared to Group P (P < 0.05). CONCLUSIONS: Midazolam potentiates the hypnotic action of propofol synergistically, but there was no evidence that the synergism extended to the blunting effect of propofol against the hypertensive response to intubation.