Summary:Five commonly used antiarrhythmic agents are examined with regard to their pharmacokinetics, pharmacology, electrophysiology, and clinical utility. The effective, nontoxic plasma levels are noted, and important drug-drug interactions with these agents are noted. Monitoring plasma levels of these agents serves a useful function in that it identifies patients who are not compliant, who are taking too little drug, or who are taking too much drug. Current techniques do not allow rapid, reproducible, and inexpensive measurements of free (vs. bound) drug in the plasma, a quantity that is likely to be substantially more valuable in determining drug effect than total plasma concentration. Previous studies on drug effectiveness may not have taken into account natural fluctuations in arrhythmia frequencies that mimic drug effect.