Phasic coronary arterial flow responses to nitroglycerin were evaluated in 15 open-chest greyhounds before and after aorta-coronary saphenous vein grafting. Coronary flow was measured with electromagnetic probes proximal and distal to the vein graft site. Simultaneous measurements of branchiocephalic flow, aortic and left atrial pressure, left ventricular pressure, and dp/dt were made. Grafts were placed during normothermic hemodilution cardiopulmonary bypass and induced ventricular fibrillation. Before grafting, intravenous nitroglycerin (0.6 mg.) elicited an abrupt rise in diastolic coronary flow which decreased as aortic pressure fell and systolic coronary flow increased. However, after grafting, nitroglycerin elicited no change in diastolic coronary flow, whereas systolic coronary flow increased and aortic pressure fell. In 6 control dogs treated similarly except for aorta-coronary grafting, coronary flow responses to nitroglycerin were unchanged. These acute experiments suggest that, in dogs with aorta-coronary saphenous vein grafts, the coronary dilating effect of nitroglycerin is virutally abolished.