We examined the efficacy of the combination of coronary reperfusion and calcium-activated neutral protease (CANP) inhibitor (E-64c) for the treatment of acute myocardial infarction in dogs. In 34 dogs, the left anterior descending artery (LAD) was occluded and reperfused after 1 hour (Groups A and B). In the remaining 49 dogs, the LAD was ligated (Groups C and D). E-64c (100 mg/kg, Groups A and C) or vehicle (Groups B and D) was injected intravenously before and after the coronary occlusion or ligation. After 24 hours the hearts were removed. The proportion of the infarct size in the LAD perfusing area (risk zone) in Group A was 47.3 +/- 9.7%, significantly lower than in Group C (54.8 +/- 8.2%, p less than 0.05) or Group D (58.7 +/- 10.0%, p less than 0.01). There was a significant difference between Group B (52.9 +/- 8.6%) and Group D as well (p less than 0.05). The effects of reperfusion (p = 0.0016) and E-64c (p = 0.0226) per se on infarct size were significant, but the combination of reperfusion and E-64c was not additive. The decrease in CPK activity in the risk zone was significantly lower in the reperfused group (p = 0.0001). The mCANP activity was higher in the border zone and lower in the infarct zone. The trend in the mu CANP activity was similar to that of mCANP. Thus, treatment with a CANP inhibitor in the early phase of acute myocardial infarction may be marginally beneficial in combination with reperfusion.