The purpose of the study was to find out the character of the restructuring of enzymes and isoenzymes of creatine kinase (CK), lactate dehydrogenase (LDH) and glucose-6-phosphate-dehydrogenase (G 6 P-DH) in the myocardium, which occurs in patients with ischaemic heart disease (IHD), and to compare the activity of the aforementioned enzymes in the myocardium of patients with IHD, examined intra vitam, and in patients who died suddenly. An analysis was made of 11 samples obtained by myocardial biopsy from the left ventricle of patients with IHD and of 11 samples obtained at autopsy of patients with IHD who have died a sudden death. Serving as controls was bioptic material from the myocardium of 12 patients without IHD. In the myocardium of patients with IHD, examined intra vitam, a decrease in the activity of the mitochondrial isoenzyme CK (MiMi) and the isoenzyme LDH-1, and a considerable rise in the activity of G 6 P-DH were found. In patients with IHD, who have died suddenly, there was found only a decrease in isoenzyme LDH-1 activity and a reduced concentration of CK B-units. The experiments proved that the differences in the myocardial isoenzyme spectrum between the two groups of patients with IHD are not connected with the process of autolysis.