The effects of myocardial reperfusion have been examined following a 1 h coronary occlusion and compared to a permanent coronary ligation in pigs. Haemodynamic investigations were carried out throughout the surgical intervention and repeated after 7 days. Cellular injury was estimated by serial serum enzyme determinations (creatin phosphokinase, alpha-hydroxybutyric dehydrogenase, aspartate aminotransferase, lactic dehydrogenase) during the first 5 postoperative days; infarct size was assessed morphometrically by a histochemical staining procedure 1 week after the temporary or permanent coronary occlusion. A linear correlation was found between the logarithmically plotted peak serum activity of AST, HBDH, CPK and the morphometrically determined infarct size. Based upon enzyme and morphometrical studies no significant difference could be detected between the two experimental groups. In the animals subjected to transient coronary occlusion, however, the development of a ventricular aneurysm had been prevented to early and sustained reperfusion. Early re-establishment of coronary circulation appears to accelerate the proliferation of a more resistant granulation tissue into the infarcted area. Cardiac performance was not improved by myocardial reperfusion.