Disopyramide exerts a quinidinelike effect on the heart and is a valuable drug for treating atrial and. especially, ventricular tachyarrhythmias. The therapeutic plasma concentration of disopyramide is thought to be 2.0–4.0 (5.0) μg/ml. We here report the cardiac effects of a high dose of disopyramide in a patient with extensive coronary artery disease complicated by ventricular extrasystoles. At plasma levels above approximately 7.0 μg/ml, heart rate was decreased, while PQ interval, QRS width, and QT interval were increased. At concentrations above approximately 6.0μg/ml. disopyramide exerted negative inotropic effects as judged by increases in mean right atrial, pulmonary arteriolar, and pulmonary capillary venous pressures and a decrease in cardiac output.