Dopamine and dobutamine are used in low output states following cardiopulmonary bypass but the consequences of increased inotropic activity on myocardium recovering from ischemia is unknown. Dogs on cardiopulmonary bypass were subjected to 20 min of normothermic global ischemia followed by 20 min of reperfusion. Dopamine or dobutamine (both at 10 μg/kg/min) or normal saline infusion was begun and 10 min later the dogs weaned from cardiopulmonary bypass while the infusions continued. Serial measurements were made of regional myocardial and systemic blood flow (15 μm radiolabeled spheres), myocardial oxygen consumption, creatine phosphate, and ATP levels. On bypass mean aortic pressure was decreased and heart rate, oxygen consumption, and left ventricular blood flow were increased by both catecholamine infusions (P < 0.01), but neither drug lowered ATP or creatine phosphate levels. Renal blood flow was decreased in dobutamine-treated dogs (P < 0.01). Off bypass, heart rate and mean aortic pressure were similar in all groups. While both drugs increased left ventricular blood flow to a similar extent (P < 0.01), dopamine treatment raised cardiac output by only 30% (P < 0.05) and dobutamine treatment increased cardiac output by 85% (P < 0.01). In addition, myocardial oxygen consumption was increased in dopamine-treated dogs (P < 0.05) while values in dobutamine animals were similar to controls. Therefore, dobutamine seems advantageous to dopamine following bypass because it increases cardiac output (by increasing stroke volume) but does not increase myocardial oxygen consumption. Both drugs are potentially detrimental on bypass because they greatly increase heart rate and oxygen consumption and, in addition, dobutamine causes an unexplained fall in renal blood flow.