The effect of nitroglycerin on total coronary flow and its regional distribution was determined in normal conditions and in myocardial ischaemia. In normal hearts, capillary flow was 9·5% higher in the inner than in the outer myocardial half. When partial occlusion of coronary arteries was followed by infusion of noradrenaline, subendocardial flow was less than capillary flow in subepicardial layers. In these conditions, the administration of nitroglycerin resulted in a redistribution of capillary flow towards normal.