Arrhythmias which last for only a few cycles or exhibit activation sequences that change from cycle to cycle may occur during the intraoperative study of ventricular tachyarrhythmias. To map these transient, varying arrhythmias requires recording from numerous sites simultaneously, which cannot be done with a single, hand-held electrode probe. Therefore, a computerized method was developed to record potentials simultaneously from up to 27 electrodes and to display epicardial and transmural activation sequences rapidly. Three arrays of electrodes are used. The electrodes of the first array are distributed over the epicardium of both ventricles and fixed within a flexible nylon mesh sock. The electrodes of the second array form a 3 cm square grid and are used to obtain a more detailed activation map of a small region of epicardium. The electrodes of the third array are spaced along the shafts of needles that are inserted through the myocardium to record transmural activation. The potentials recorded from an array of electrodes and the computer-selected activation times at the electrodes are displayed for visual inspection after which a map of activation is drawn. The activation sequence of a single cycle can be displayed 10 minutes after recording the potentials. The activation map obtained from the shock array determines the placement of the grid array; the map resulting from the grid array determines the placement of the needle array. Thus, the region responsible for initiating an arrhythmia can be determined if the arrhythmia can be induced three or more times.