The distribution of coronary flow during high-flow hypoxia and ischemia has been assessed in the perfused rat heart. Surface and cross-section NADH fluorescence photography defined the pattern of oxygen delivery because regions of high NADH fluorescence denote areas of anoxia. Thioflavin S, a fluorescent dye that stains vascular endothelium, was added to the perfusate to label perfused vessels. The results indicate 1) that relatively large anoxic zones develop during high-flow hypoxia or ischemia, 2) that the pattern of oxygen delivery under hypoxic conditions is reproducible, and 3) that flow into these anaerobic areas is significantly decreased relative to control. The border zone between aerobic and anoxic tissue is shown to be sharply defined, indicating the absence of an appreciable volume of tissue that is partially anaerobic. The data may be relevant to human coronary artery disease, where coronary occlusion is usuallly not complete. They suggest that with reduced, but not insignificant, oxygen delivery into the domain of a coronary artery, the distribution of flow would be heterogeneous, making small areas of tissue severely ischemic while preserving flow and oxygen supply in the adjacent tissue.