Calcium antagonists have emerged as a new class of antiarrhythmic agents for the control of certain supraventricular and ventricular arrhythmias. Electrophysiologically, these agents are heterogeneous but their main action is mediated through a depressant effect on the slow calcium channel in cardiac muscle, most readily demonstrated in isolated tissue preparations. In vivo, their actions are modulated by their reflex actions and by their interaction with the autonomic nervous system due to the noncompetitive adrenergic-blocking actions that some of the compounds exhibit. The major agents exerting antiarrhythmic actions are verapamil, diltiazem, gallopamil, tiapamil and bepridil; the dihydropyridines are devoid of electrophysiologic actions in vivo. Calcium antagonists prolong intranodal conduction time, lengthen the effective and functional refractory periods in the atrioventricular node but exert little or no effect on atrial, ventricular, His-Purkinje or bypass tract conduction or refractoriness (except in the case of bepridil, which has additional electrophysiologic properties). These effects form the basis of the clinical antiarrhythmic effects of this class of agents. The most striking action is the predictable and prompt termination of the reentrant supraventricular tachycardia by intravenous verapamil and diltiazem and the slowing of the ventricular response in atrial flutter and fibrillation. These agents may also be of value in the long-term control of ventricular response in atrial flutter and fibrillation; their role in multifocal atrial tachycardia and other ectopic tachycardias is less well defined. Calcium antagonists reverse ischemic ventricular arrhythmias caused by coronary artery spasm but exert little or no action in the usual forms of sustained ventricular tachyarrhythmias associated with severe structural heart disease. They are poor suppressants of ventricular premature complexes. Recent data have established their role in exercise-induced tachycardia occurring in the context of ischemic heart disease; they are also of value in ventricular tachycardia occurring in young patients who develop tachycardia with a right bundle branch block and left axis deviation morphology, an arrhythmia thought to be due to triggered automaticity.