A video and multiplexing system is described that provides a source of display in real-time, slow-motion, and stop-frame modes of ventricular activation. The displayed image, consisting of a matrix of 112 dots, is arranged to represent a polar projection of the ventricles. The dots brighten at the time of local cardiac activation, which is sensed by an electrode array monitoring the myocardium. Experiments in in situ canine hearts have provided detailed depictions at 8.3-ms intervals of the ventricular activation sequence during sinus and ventricularly paced rhythm. The origin of ventricular paced beats can be located with an accuracy of less than 5 mm. Coronary occlusion produces a defect in the displayed image that outlines the ischemic area and demonstrates regions of delayed conduction. This approach to ventricular mapping provides a detailed on-line display of activation with a recording capacity of 1 h. The system offers a practical, inexpensive alternative to computerized methods of investigating ventricular arrhythmias in both the experimental and intraoperative environment.