The relationship of changes in regional coronary flow to the nature and degree of biochemical disturbances during occlusion of branches of the left anterior descending coronary artery and following reestablishment of flow was investigated in two groups of dogs: group I, moderate ischemia before reflow, and group II, severe ischemia prior to reflow. Regional coronary blood flow was determined before ligation, after 60 min of ischemia and after 15 min of reflow using labelled microspheres. Hearts made ischemic for 60 min but not reperfused served as controls. Groups I and II were distinguished by the following features. Group II showed a marked exacerbation of biochemical damage on reperfusion of the ischemic region (reduced levels of ATP, impairment of mitoehondrial oxygen consumption and mitoehondrial calcium binding). This was accompanied by significant subendocardial hyperemia. Reperfusion in group I, on the other hand, partially reversed these changes (increased level of ATP in the ischemic-reperfused region, improved mitoehondrial oxygen consumption and calcium binding). Mitoehondrial calcium uptake and oxidative phosphorylation (ADP/O ratio) were not affected in any group. These data illustrate that the degree of biochemical damage following reperfusion of the ischemic myocardium is determined by the degree of ischemia, and suggest that interference with ATP production by the mitochondria is not responsible for the damage.