In order to highlight severity of myocardial injury during the course of global ischemia and reperfusion, cytochemistry of glycogen and succinate dehydrogenase (SDH) as well as hematoxylin and eosin staining (H & E) and electron microscopy were observed in canine myocardium. Seven mongrel dogs were selected for reperfusion injury after global ischemia in this study. Myocardial biopsies were taken from the anterior wall of the left ventricle (a) after cardiopulmonary bypass (the first biopsy); (b) at the end of the aortic crossclamp (the second biopsy); and (c) 30 minutes after crossclamp removal (the third biopsy). All biopsies were cytochemically assessed, and the latter two, for electron microscopic studies. The averages of myocardial necrotic rate and surface to volume ratio of myocardial mitochondria were calculated under electron microscopy and in electron microscopic slices, respectively. Myofibrillae were of normal morphology in the first biopsy; in wave-shape and partly vacuolated, with large and deformed nuclei in the second one; and in wave-shape and severely vacuolated in the third one, in H & E. Glycogen granules were variously stained in moderate, weak and intensive positive reactions in the three biopsies respectively in glycogen staining. SDH was stained in intensive, weak, and moderate positive reactions in three, respectively. By electron microscopy, Z bands twisted severely, and local dissolution of cristae and matrix occurred in a minority of the mitochondria in the second biopsy; and majority of the Z bands in necrotic region had disappeared, the myofibrillae were obscure and patchily dissolved. Clustered and deformed mitochondria could be found in the third biopsy. Significant difference could be noted between the averages of the second and third biopsies (14.88 +/- 3.09% vs. 60.25 +/- 8.55%, p < 0.001). The surface to volume ratio of the ischemic mitochondria was much bigger than that of the reperfused (3.95 +/- 1.09 micron-1 vs. 2.77 +/- 0.93 micron-1, p = 0.041). Myocardial injury was more severe in reperfusion than in ischemia myocardium. There were correlations between histobiochemical and ultrastructural alterations in damaged canine myocardium.