The circulatory effects of anileridine, a derivative of pethidine, have been little studied. Therefore we compared the haemodynamic effects of equianalgesic doses of pethidine (1 mg/kg i.v.) and anileridine (0.25 mg/kg) in matched patients requiring myocardial revascularization. Cardiac output was significantly increased 5 min after the administration of pethidine, mainly due to an increase in heart rate. A transient rise in the systolic pulmonary-arterial pressure was found after anileridine. No remarkable changes were found in systemic arterial pressures, central venous pressure, balloon-occluded pulmonary-arterial pressure, stoke volume, systemic or pulmonary vascular resistances and derived oxygen consumption. Further, 20 min after drug administration, there were no significant differences in any circulatory parameters between the two groups. One patient developed acute cardiac failure after anileridine, though as he had very severe coronary heart disease it remains an open question whether this was spontaneous or drug-induced. Since the rate-pressure product tended to increase after pethidine, this drug may not be considered an ideal analgesic for patients with ischaemic heart disease. Anileridine had less influence on this variable. Since the circulatory effects of pethidine seem to depend on the haemodynamic status of the patient, the haemodynamic properties of anileridine may also deserve further investigation.