The performance of a commercial ambulatory electrocardiography analyzer was evaluated when used for the quantitative analysis of arrhythmias in an antiarrhythmic research investigation. The detection and classification of the QRS complexes were examined. The variation between the edited trend report obtained in the investigation and the true arrhythmia quantities established by visual counting was determined for ventricular and atrial premature beats. The analyzer system adequately fulfilled the demands of the research program in establishing changes in the frequency of the arrhythmias.