Electrocardiographic monitoring can be a useful adjunct to antiarrhythmic therapy, since the electrocardiogram is a simple indicator of net cardiac drug effect, irrespective of factors such as pharmacokinetic variability, drug-metabolite interactions or intraindividual variability in drug sensitivity. Changes associated with antiarrhythmic drug therapy include markers of sodium channel block, such as increased QRS or sinus-ectopic coupling intervals and increased QT interval, a marker of action potential prolongation. Electrocardiographic changes can serve 3 purposes: They can correlate with arrhythmia suppression, they may be a guide to impending drug toxicity, and they can indicate the presence of an antiarrhythmic drug at some electrophysiologically active site in the heart. This latter indication may be used as an assessment of compliance, as a clue to drug-drug interactions that may lower antiarrhythmic drug concentrations or preparatory to electrophysiologic testing when it is desirable to avoid testing patients who have no demonstrable drug effect. Drug-induced changes in the microelectrophysiologic environment may sometimes fail to express themselves on the surface electrocardiogram. Overall, however, the electrocardiogram is an inexpensive, readily available tool to monitor net antiarrhythmic drug effects on the heart. Monitoring of the electrocardiogram should, therefore, be an integral part of managing antiarrhythmic drug therapy in patients with arrhythmias.