Great advances have been made in our understanding of cardiac arrhythmias and the effect of drugs on these arrhythmias. Several new types of antiarrhythmic drugs have been introduced. For example, encainide, flecainide, or lorcainide are new type I drugs that exert their major effects on depressing cardiac conduction. These drugs appear to be very effective for suppression of premature ventricular contractions. The introduction of verapamil (a calcium antagonist) allows for rapid control of supraventricular arrhythmias by virtue of its effects on the slow-channel (predominately Ca++) receptors. New beta-blocking drugs have been introduced which may prove beneficial in certain clinical situations since they exert more selective blockade of the cardiac receptors (beta1) as opposed to smooth muscle receptors (beta2). Amiodarone is an extremely potent drug which is effective for both supraventricular as well as ventricular arrhythmias. The major cellular electrophysiologic effect of this drug is to prolong the action potential duration. Its major clinical drawback is the rather formidable long-term side effects which include corneal opacities, thyroid dysfunction, neurologic toxicity, and pulmonary fibrosis.