Long-term continuous electrocardiographic recordings (Holter recordings) and electrophysiologic testing are useful for selecting patients for antiarrhythmic drug trials and for evaluating efficacy and adverse effects during therapy. These 2 methods are used to establish patient eligibility and to stratify patients during randomization. Both noninvasive testing and electrophysiologic studies help to classify arrhythmias as benign, potentially malignant or malignant. Holter monitoring and electrophysiologic studies each have unique advantages and disadvantages for baseline evaluation before starting antiarrhythmic drug treatment and for evaluation of efficacy or adverse effects during follow-up. Both methods have been shown to predict outcome of treatment in patients with malignant ventricular arrhythmias (i.e., can be used as surrogates for sudden death). Several ongoing studies are attempting to extend our knowledge of these 2 techniques. A multicenter study in the United States is comparing the 2 methods for applicability, predictive accuracy and cost. Investigators in the Netherlands are testing the validity of electrophysiologic studies by continuing antiarrhythmic drug treatment whether or not programmed ventricular stimulation predicts success or failure. Finally, new proposals have been made for conducting randomized, controlled studies in selected patients with malignant ventricular arrhythmias using time to drug failure as the endpoint.