Plasma concentration monitoring of antiarrhythmic agents is valuable, but it is often misused or overemphasized in therapeutic decision-making. There are strict requirements for its appropriate use that are often not met--for both the newer and even the conventional antiarrhythmic drugs. For maximum value, there must be a reliable, accurate relation between the plasma drug concentration and drug action, a relation closer than that between dosage and drug action. The time of sample collection is important--most guidelines are based on "trough" plasma concentrations measured after steady-state equilibrium has been achieved. The use of an accurate, sensitive and specific assay is crucial to the value of plasma concentration monitoring guidelines. However, for agents having active metabolites, monitoring the concentration of only the parent drug can be misleading and limits (but does not necessarily eliminate) the value of plasma concentration monitoring guidelines for these agents. Plasma concentration monitoring of most antiarrhythmic agents is of value for certain specific purposes: to determine compliance to antiarrhythmic therapy, to detect and analyze possible drug interactions, to assess the benefit to risk ratio for increasing the dose of a particular antiarrhythmic agent, to maintain a stable drug effect in the presence of a patient's changing clinical condition and, to a limited extent, to assess the role of an agent in causing an adverse drug reaction. The importance of understanding the assay methods currently in use, as well as how plasma concentration monitoring of individual antiarrhythmic agents is affected by the presence of active metabolites, optical isomers differing in their activity and variations in protein binding, is essential in interpreting data obtained from plasma concentration monitoring.