Subendocardial ischemia without anatomic coronary artery obstruction may result from a discrepancy between metabolic needs and available blood supply. We studied this in open-chest anesthetized dogs and measured pressures in aorta and left ventricle (LV), phasic left coronary arterial blood flow (CBF) by electromagnetic flowmeter, total CBF and LV subendocardial (endo) and subepicardial (epi) flow with radioactive microspheres 8−10μ, in diameter. Since LV Subendocardial flow is mainly or entirely diastolic, it should depend on coronary driving pressure and duration of diastole (i.e., the area between aortic and left ventricular diastolic pressures). This diastolic pressure time index (DPTI) was varied by opening arteriovenous fistulas to lower aortic diastolic pressure, constricting the ascending aorta to raise LV diastolic pressure and pacing to shorten diastole. Myocardial oxygen needs were estimated from the tension time index (TTI). Normal endo-epi flow ratios per gram (1:1) fell to 0.1:1 with these procedures and paralleled a fall in diastolic flow fraction (often nearly zero) and postischemic coronary reactive hyperemic responses. These changes occurred despite normal or raised mean CBF and 300−500% increase in systolic CBF. The altered flow ratios were best predicted by relating them to the ratio of DPTI (supply) to TTI (demand).