The new water-soluble benzodiazepine midazolam was compared in a randomized study to diazepam for postoperative sedation in fifty patients following aortocoronary bypass surgery with a sufentanil-anesthesia. Midazolam and diazepam were administered intravenously in repeated doses in conjunction with an opioid infusion (piritramide) from the end of surgery during a twelve-hour study period, patients being artificially ventilated. Midazolam scored better than diazepam for quality of sedation and cardiovascular stability during the period of mechanical ventilation and for respiration during the weaning period and after extubation, although no difference was found in weaning time from artificial ventilation and time of extubation. Hemodynamic tolerance for both drugs was good. The administration of a loading dose of midazolam 5 mg caused a slight, transient decrease in mean arterial pressure. Midazolam appeared to be a more effective sedative agent than diazepam for short-term administration during mechanical ventilation. No evidence of cumulation and prolonged recovery was seen.