This study was designed to quantify and compare transmural myocardial blood flow in the right and left ventricles of awake dogs during systole and diastole, and during total coronary artery occlusion. Studies were performed in six awake dogs chronically prepared with electromagnetic flowmeters and pneumatic occluders on the left circumflex and right coronary arteries. Intermittent coronary perfusion, confined to the interval of cardiac systole or an equivalent period during diastole, was effected by an R wave-triggered pneumatic valve connected to the occluders. To measure regional myocardial blood flow, radionuclide-labelled microspheres, 7-10μm in diameter, were injected into the left atrium. The authors found that when arterial inflow was confined to systole, blood flow was normal in the left ventricular subepicardium, whereas flow to deeper myocardial layers was decreased as a linear function of tissue depth. When coronary arterial inflow was limited to an equivalent period in diastole, left ventricular transmural flow was uniform. In contrast to this, right ventricular transmural myocardial blood flow was unchanged from control values when coronary inflow was confined to systole or to an equivalent interval in diastole. During total coronary artery occlusion, absolute blood flow decreased similarly in myocardial areas perfused by the left circumflex and right coronary arteries. However, the transmural distribution of collateral blood flow was markedly different in the two ventricles. Left ventricular collateral flow was preferentially directed to the subepicardial layers, but right ventricular collateral flow was distributed uniformly across the ventricular wall. The transmural gradients observed in the left ventricle during total coronary artery occlusion and when arterial inflow was confined to systole can be explained by the functioning of a vascular waterfall during left ventricular contraction.