The subendocardium is the most vulnerable area of the left ventricle to the effects of hypoperfusion and ischemia. Despite this well acknowledged observation, the mechanisms underlying this susceptibility are not elucidated, although numerous explanations including differences in transmural distribution of hemodynamics, metabolism, and wall stresses have been proposed. Our goal was to make dynamic measurements of endocardial and epicardial flow velocities, which reflect hemodynamic and wall stresses, to approach this problem. Therefore, we measured blood flow velocities in subendocardial and subepicardial coronary arterioles of in vivo beating canine heart using a high-speed CCD intravital videomicroscope with rod-probe lens. Subendocardial flow was characterized by remarkable systolic flow velocity reversal (systolic slosh ratio: 84%, measurable velocity of retrograde flow was faster than -40 mm/sec), which contrasted to predominant forward flow velocity during systole in the subepicardial arterioles (systolic slosh ratio: 25%, maximum velocity: ~ -20 mm/sec, P < 0.0005 and P <0.05 vs. subendocardial arterioles, respectively). We speculate that this retrograde flow is "wasteful" because this volume must be refilled during the following diastole; thereby detracting from the net perfusion as well as the time for perfusion. Accordingly, we also believe that the retrograde systolic blood flow contributes to the vulnerability of the subendocardium to ischemia. he subendocardium is the most vulnerable area of the left ventricle to the effects of hypoperfusion and ischemia. Despite this well acknowledged observation, the mechanisms underlying this susceptibility are not elucidated, although numerous explanations including differences in transmural distribution of hemodynamics, metabolism, and wall stresses have been proposed. Our goal was to make dynamic measurements of endocardial and epicardial flow velocities, which reflect hemodynamic and wall stresses, to approach this problem. Therefore, we measured blood flow velocities in subendocardial and subepicardial coronary arterioles of in vivo beating canine heart using a high-speed CCD intravital videomicroscope with rod-probe lens. Subendocardial flow was characterized by remarkable systolic flow velocity reversal (systolic slosh ratio: 84%, measurable velocity of retrograde flow was faster than -40 mm/sec), which contrasted to predominant forward flow velocity during systole in the subepicardial arterioles (systolic slosh ratio: 25%, maximum velocity: ~ -20 mm/sec, P < 0.0005 and P < 0.05 vs. subendocardial arterioles, respectively). We speculate that this retrograde flow is "wasteful" because this volume must be refilled during the following diastole; thereby detracting from the net perfusion as well as the time for perfusion. Accordingly, we also believe that the retrograde systolic blood flow contributes to the vulnerability of the subendocardium to ischemia.