The purpose of this study was to determine whether depletion of serum complement can decrease the severity of an ischemic episode by improving regional O2 supply and consumption parameters in the ischemic region of the heart. Fourteen anesthetized dogs with serum complement intact or depleted (100 U/kg cobra venom factor given 8 hrs before) were subjected to left anterior descending coronary artery (LAD) occlusion for 6 hrs. Myocardial blood flows were determined before and 6 hrs after LAD occlusion using radioactive microspheres. Regional arterial and venous O2 saturations were determined using microspectrophotometry. In control animals, flow decreased from 122 +/- 42 to 13 +/- 14 ml/min/100 g (mean +/- SD) in the occluded LAD region. With complement depletion, LAD occlusion resulted in a flow reduction in the ischemic region (38 +/- 29 ml/min/100 g), but to a lesser degree than seen in the same region in control animals, especially in the subendocardium. O2 consumption was decreased in the ischemic region of both treatment groups, though O2 consumption was higher in this region in complement depleted animals compared to the values in control animals. The O2 supply/consumption ratio was decreased similarly in the ischemic region of control and complement depleted groups. Thus, with complement depletion, flow to the ischemic zone was improved but this region was still flow restricted. The flow increase during complement depletion was sufficient to allow an increased O2 utilization in the ischemic region.