Most patients requiring antiarrhythmic therapy for ventricular arrhythmias have underlying organic heart disease which causes left ventricular dysfunction. Treatment of these patients' arrhythmias requires knowledge of the hemodynamic as well as antiarrhythmic effects of the available agents. These effects may differ during acute and chronic oral therapy. Several of the newer agents and quinidine are very effective in suppressing ventricular ectopic activity, allowing demonstration of drug effect during ambulatory monitoring. The clinical significance of this for prevention of sudden death has yet to be shown. However, prevention of ventricular tachycardia or fibrillation in the electrophysiology laboratory and suppression of ambient ectopy may generally be separate phenomena, as has been demonstrated for amiodarone.