By the use of a precise and sensitive double-isotope derivative technic, plasma catecholamine concentration was measured at 2-hour intervals in 10 patients during the first 48 hours of myocardial infarction.Plasma catecholamine concentration was elevated in most patients, but to an extremely variable degree. In each patient, however, the values were rather stable during the study period. High levels of plasma catecholamines were correlated to the clinical state of the patients. Patients who received antiarrhythmic treatment on admission to the hospital had higher values than untreated patients. There was no temporal correlation between plasma catecholamine concentration and ventricular arrhythmias. In contrast to the plasma catecholamine level, the heterotopic ventricular activity declined spontaneously in the untreated patients within the study period.It is emphasized that the plasma catecholamine concentration in patients with acute myocardial infarction is considerably lower than the level necessary to produce arrhythmias in experimental myocardial infarction.