Effects of perfusion pressure augmentation with methoxamine on transmural distribution and adequacy of coronary blood flow during cardiopulmonary bypass were investigated. Flow in normal, nonworking canine hearts was measured with tracer microspheres and electromagnetic flow probes while heart rate, myocardial oxygen consumption, left ventricular contractility and epicardial ST-segment stability were monitored. Measurements during normotensive cardiopulmonary bypass were compared with values during intraoperative hypotension and subsequent normotension achieved following methoxamine infusion. Total and regional coronary blood flow returned to levels not different from normotensive controls after methoxamine infusion with a redistribution of flow to the subendocardium. We conclude that elevating perfusion pressure during cardiopulmonary bypass with methoxamine infusion increases total coronary and subendocardial blood flow by means of peripheral and selective subepicardial alpha adrenergic vasoconstriction.