30 cardiac surgical patients were investigated for comparing the respiratory and haemodynamic repercussions of induction of anaesthesia with either flunitrazepam or 8-chloro-6-(2-fluorophenyl)-1-methyl-4H-imidazo[1,5-a] [1,4]benzodiazepine (midazolam, Ro 21-3981, Dormicum). The patients were premedicated with morphine, 0.15 mg/kg i.m., and divided into two groups: one group receiving flunitrazepam 0.03 mg/kg, the other midazolam 0.15 mg/kg injected i.v. over a 15-s period. The induction time and the respiratory depression were similar in both groups. A similar decrease in systolic, diastolic and mean arterial pressures was noted, with maximum decrease of mean arterial pressure of 21% 5 min after injection of flunitrazepam, and of 19% after injection of midazolam. Midazolam produced a significant but transient pulse acceleration of 4% 1 min after the injection, while flunitrazepam produced a moderate decrease of the heart rate. These results indicate that in cardiac surgical patients midazolam produces a moderate respiratory and cardiovascular depression similar to flunitrazepam.