The haemodynamic effects of dobutamine (2 microgram/kg . min and 4 microgram/kg . min) and dopamine (4 microgram/kg . min and 8 microgram/kg . min) were studied in 17 patients with coronary artery disease prior to coronary bypass surgery. The study was performed under general anaesthesia (modified neurolept analgesia) and controlled ventilation. Dopamine improved cardiac index significantly, increased mean aortic pressure slightly while heart rate and total peripheral resistance remained unchanged. Dobutamine failed to increase cardiac and stroke index significantly, but increased mean aortic pressure distinctly due to an elevated total peripheral resistance. Both catecholamines increased left ventricular filling and mean pulmonary artery pressure. The HR x ASP-product which is closely related to left ventricular oxygen consumption was found to be augmented to a greater extent during dobutamine. For the above reasons dopamine should be favoured for increasing cardiac output in patients undergoing aortocoronary bypass surgery. Our study does not confirm earlier results which have shown dobutamine to be the preferable catecholamine. The possible reasons for this discrepancy are discussed.