In spite of the ratio between plasmatic concentration and antiarrhythmic effects of a drug many factors can be influenced by many factors, the plasma level monitoring plays an important clinical role. The efficacy of the antiarrhythmic therapy should be evaluated not only by means of ECG-Holter monitoring and/or electrophysiological study but also by the means of determination of plasmatic and tissular concentration of a drug and metabolites. This latter is indispensable in the evaluation of new antiarrhythmic drugs and of a dose/effect relationship during acute and chronic therapy.