Phasic coronary flow and transmural myocardial perfusion were studied during coronary artery constriction in 10 dogs with electromagnetic flowmeters and hydraulic occluders on the left circumflex coronary artery. Regional blood flow was estimated using left atrial injections of radionuclide-labeled microspheres 9 μm in diameter. During control conditions, 14 ± 0.7% of coronary flow occurred in systole and subendocardial (endo) flow exceeded subepicardial (epi) flow in the region of the posterior papillary muscle (endo/epi = 1.16). Immediately following partial inflation of the occluder to reduce coronary inflow 39 ± 3%, systolic flow was unchanged while diastolic flow fell 42 ± 3%, thereby increasing the portion of flow in systole to 22 ± 1.4%. At this time blood flow was reduced nearly uniformly across the wall of the left ventricle. When coronary constriction was maintained for 30 s, an additional shift of flow from diastole to systole occurred (systolic flow = 29 ± 2.3% of total inflow), and transmural perfusion was redistributed from subendocardium to subepicardium (endo/epi = 0.37). This redistribution of flow was accompanied by a fall in coronary pressure distal to the occluder, indicating vasodilation of the distal coronary vasculature.