The haemodynamic effects of dopamine and dobutamine were compared in a cross-over study of 12 patients in the early postoperative phase after open-heart surgery. The drug infusion rates (dopamine (microgram/kg/min) mean 6.5, range 2.8-12, dobutamine (microgram/kg/min) mean 7.9, range 4.3-12.3) were adjusted so that the cardiac output increased by 50%. With both drugs this was achieved through simultaneous increases in stroke volume (dopamine + 16%, dobutamine + 9%) and heart rate (dopamine + 31%, dobutamine + 38%). The systemic vascular resistance did not change with dopamine but decreased significantly (-18%) with dobutamine. Therefore, the systolic and diastolic arterial blood pressures rose significantly more with dopamine than with dobutamine. The left atrial pressure increased with dopamine but was unchanged with dobutamine. The urine output was significantly higher with dopamine than with dobutamine.