The influence of several intravenous anaesthetics on the heart was assessed using the isolated rat heart-lung preparation. Each group received 10(-3) mol litre-1 and 10(-4) mol litre-1 of ketamine, 6 mug ml-1 and 60 pg ml-1 of midazolam, 6 mug ml-1 and 60 mug ml-1 of diazepam or 0.6 mug ml-1 and 6 mug ml-1 of flunitrazepam. Systolic blood pressure in rats receiving high doses of midazolam, diazepam and ketamine were higher than that in the control group. Heart rate in rats receiving high doses of ketamine and diazepam were lower than that in the control group. However, there were no significant changes in cardiac output among the groups. Maximum rate of left ventricular tension development in rats receiving high doses of midazolam and diazepam increased significantly in comparison with that in the control group. There were no significant changes in myocardial high energy phosphates among the groups. None of the intravenous anaesthetics, even in doses which were 100 times greater than therapeutic doses, showed any depressant effects in this preparation. Moreover, it is surprising that midazolam and diazepam produced direct increases in myocardial contractility. These results suggest that the cardiodepressant effects of intravenous anaesthetics may be due to their effects on the central nervous system.