A computer system has been developed to study the sequence of ventricular activation during surgery. The system consists of a set of epicardial and intramural electrodes for detecting the cardiac signals, a 32-channel analog tape recorder, a 32-channel analog-to-digital converter interfaced to a Digital Equipment Corporation PDP 11/34 computer, and a Tektronix 4014 graphics terminal. The electrodes are attached to a sock that can be slipped over the ventricles allowing the electrodes to contact the epicardium. Also used a plunge electrodes for the measurement of myocardial and cavity potentials. The signals are either digitized directly from the electrodes or digitized on playback from the analog tape. Typically the signals are sampled continuously at a 1 KHz sampling rate and stored in a circular buffer on a disk. In this way the preceding 38 seconds of data are available for analysis. This system was designed to be used in the surgical treatment of tachyarrhythmias, especially complex ventricular dysrhythmias unreponsive to medical therapy.