The activity of glutamino-oxaloacetic transaminase was studied in 85 and that of creatinophosphokinase, total lactate-dehydrogenase and its isoenzymes in 58 patients with macrofocal myocardial infarction complicated by a shock. The maximal transaminase activity in patients with myocardial infarction attended by a shock was on the average greater than in similar patients without any shock. In patients with myocardial infarction attended by a shock who presented, at the same time, congestive manifestations in the systemic ciruclation demonstrated a higher transaminase activity and its normalization at a later date than this was in the case of patients with a shock, but without congestion in the systemic circulation. In patients suffering from myocardial infarction with a shock the activity of creatinophosphokinase and total lactate-dehydrogenase averaged higher than in those without shock. In the blood serum of such patients the proportions of the lactate-dehydrogenase isoenzymes were found to undergo substantial changes, which consisted in an increased activity of the LDH1 and LDH5 fractions. The main factor responsible for a considerable rise in the activity of the blood serum transaminase, creatinophosphokinase, total lactate-dehydrogenase and the LDH1 fraction occurring in patients with myocardial infarction attended by a shock is the great extent of the necrotic focus in the myocardium.