For more than 30 years, the monophasic action potential has been used as an experimental tool for the study of myocardial repolarization. With recent improvements in catheter design, the utility of the tool as a means to identify the bases for ventricular arrhythmias in humans has been greatly improved. Abnormalities of repolarization leading to ventricular arrhythmia formation can be identified and specific pharmacologic therapies may be evaluated. Further evaluation of the technique by cardiac electrophysiologists may improve both the diagnosis and the treatment of ventricular arrhythmias dependent upon afterdepolarization formation (acquired and familial long QT syndromes).