Because of clinical observations suggesting that nitroglycerin may suppress premature ventricular complexes during acute ischaemia, a study was undertaken to assess the effect of nitroglycerin on the incidence of premature ventricular complexes in patients with acute myocardial infarction. Forty patients with acute myocardial infarction were studied. Twenty-six patients received 0.4 mg nitroglycerin sublingually every 4 hours for the first 24 hours after admission to the coronary care unit. The total premature ventricular complex count for the 26 patients for 15 minutes before nitroglycerin was 592, and 276 for the 15 minutes after the drug (P less than 0.005). In the remaining 14 patients on the same nitroglycerin schedule, a single electrocardiographic lead was continuously recorded on tape. During the first hour after nitroglycerin, the premature ventricular complex count decreased by 58 per cent, and the second and third hours showed a decrease from control count of 71 and 65 per cent respectively. By the end of 4 hours the ectopic count was back to control level. The data indicate that nitroglycerin may decrease the number of premature ventricular complexes for up to 3 hours in patients with acute myocardial infarction. The mechanism of action of nitroglycerin is not elucidated by this study, but the observation may be of value in further studies of specific antiarrhythmic therapy and prevention of arrhythmias in patients with coronary artery disease.