Objective-In order to investigate the salvage of ischaemic myocardium by polyethylene glycol-conjugated superoxide dismutase (PEG-SOD), we compared reperfused and non-reperfused regions in the same canine heart and measured regional myocardial blood flow (RMBF) and myocardial CPK during coronary occlusion and reperfusion using non-radioactive, coloured microspheres. Methods and results-The chests of 17 mongrel dogs were opened under anaesthesia, and the left circumflex coronary artery was occluded for 90 min and then reperfused for 5 min. During this procedure, polystyrene microspheres of different colours were infused at four different times: prior to occlusion (orange), 10 min (red) and 90 min (blue) after occlusion, and 5 min after reperfusion (yellow). Thereafter, the heart was excised, cut in slices along the left circumflex coronary artery, and flow rates at the various times were assessed as a function of microsphere counts. In the control group (n = 9), there are significant differences in the myocardial CPK level between reperfused and non-reperfused areas. The myocardial CPK level in reperfused area was significantly reduced compared to non-reperfused area in the outer layers (54 +/- 8 IU/g vs. 74 +/- 9 IU/g, P < 0.05), and also reduced in the inner layers (59 +/- 9 IU/g vs. 74 +/- 13 IU/g), however, it was not significantly different. In the PEG-SOD group (n = 8), there was no significant difference in the myocardial CPK level between reperfused and non-reperfused areas in both inner and outer layers (inner layers; 68 +/- 11 IU/g vs. 68 +/- 6 IU/g, outer layer; 69 +/- 17 IU/g vs. 67 +/- 18 IU/g), indicating a significant protective effect of PEG-SOD. In the control group, transmural necrosis of the reperfused areas was 22.4 +/- 10.0%, which showed no significant difference compared with non-reperfused areas (23.1 +/- 9.9%). In the PEG-SOD group, transmural necrosis of the reperfused areas by TTC staining was 8.1 +/- 8.1%, which showed no significant difference compared with non-reperfused areas (8.5 +/- 7.1%). Conclusions-PEG-SOD prevents infarct extension during early coronary reperfusion.