The incidence of cardiac arrhythmias increase with aging. Generally, in the elderly, serum drug concentration is apt to increase rapidly and maintained at the high level for a longer time because of the reduced liver and kidney functions. Thus, the efficacy and side effect are both apt to occur rapidly and for a long time. In the elderly, specialized conduction system including sinus node and atrioventricular node is often impaired, and, bradycardia, widening of the QRS duration and prolongation of the QT interval are often induced by antiarrhythmic drugs. The elderly should be treated initially at a lower dosage than usual and be followed-up by observing serum drug concentration to obtain the efficacy without serious adverse effect.