In spontaneously breathing rats, continuous infusion of etomidate with and without fentanyl caused a slight decrease in blood pressure and heart rate. Coadministration of fentanyl and etomidate in order to obtain full anaesthesia and analgesia resulted in respiratory depression. In artificially ventilated rats both etomidate as well as the anaesthetic combination caused a strong reduction in aortic flow and an increase in total peripheral resistance. A single infusion of etomidate did not change blood pressure. Etomidate combined with fentanyl reduced blood pressure. Under adjusted ventilation blood pressure, aortic flow, max(dF/dt) and heart rate were progressively reduced during a 4 h period. In contrast, urethane anaesthesia reduced aortic flow to a minor extent. Total peripheral resistance and max(dF/dt) were hardly affected. The slightly reduced blood pressure and blood gas variables remained stable during the experiment. From pharmacokinetic studies it was established that effective etomidate plasma levels were maintained constant during the experimental period. Pharmacokinetic interaction between etomidate and fentanyl did not occur. It is concluded that for anaesthesia of longer duration during cardiovascular experiments in rats, urethane is preferable to etomidate/fentanyl because it does not cause serious changes in basal haemodynamic variables.