Hemodynamic changes after single intravenous injection of antiarrhythmic doses of propafenone (70 mg) and lidocaine (100 mg) were measured comparatively in 11 patients with acute myocardial infarction, stable cardiac rhythm and without evidence of manifest left heart failure. The effects of propafenone were characterized by a significant decrease of cardiac index by 6% and an increase in left and right ventricular filling pressures by 15% and 23%. Mean arterial pressure was significantly lowered (8%) and left ventricular stroke work index decreased by an average of 11%. Mean pulmonary artery pressure significantly increased by 7%. A fall in systemic vascular resistance by 7% was not statistically significant. Intravenous administration of propafenone at a dose of 70 mg thus caused a transitory decrease in ventricular function and acute vasodilation with a fall in systemic blood pressure. The intravenous administration of lidocaine did not result in significant changes of cardiac index or left and right ventricular filling pressures. Mean pulmonary artery pressure, systemic blood pressure and vascular resistance increased significantly.