A portable isopotential surface mapping system was developed to determine if body surface maps can be measured as an integral part of diagnostic studies in acutely ill patients and in those undergoing electrophysiological cardiac catheterization for arrhythmias. In patients with WPW syndrome the recording of body surface maps was helpful when there was electrical drive of the atrium (via an intracardiac catheter) to maximize preexcitation. Either increasing the atrial rate or stimulating the left atrium directly enhanced ventricular preexcitation for recording maps in this group of patients. Predictions of the preexcitation site from body surface maps were based on intact chimpanzee experiments. The results were compared with the location of the preexcitation sites that were documented at surgery. There was good agreement between predicted and actual sites of the bypass tracts. The results indicate that the inclusion of body surface maps as an integral component of electrophysiological catheterization studies in patients with arrhythmias is useful when there is a need to know the location of the site of onset of ventricular activation.