Excitation in the epicardial border zone of 3-5-day-old canine infarcts was mapped with an array of 192 bipolar electrodes during sustained ventricular tachycardia. Reentrant circuits were found in which activation occurred around long lines of apparent conduction block based on the criterion that excitation on opposite sides of the lines occurred with marked disparity in time. When the lines of apparent block were functional (i.e., occurred only during tachycardia and not during sinus rhythm or ventricular pacing) they were oriented parallel to the long axis of epicardial muscle fiber bundles. Isochrones distal to the lines were oriented parallel to them because widely separate sites within these isochrones were activated nearly simultaneously. This suggested that excitation not only occurred around the lines of block but also slowly across them. This slow activation occurred transverse to the long axis of the myocardial fibers and therefore might result because of the anisotropic tissue properties. To test this hypothesis, the epicardial border zone was stimulated during sinus rhythm through electrodes around its margin and at the center of the recording array. Activation transverse to the myocardial fibers in regions where lines of block occurred during tachycardia was slow, whereas it was rapid parallel to fibers' orientation. During tachycardia electrograms along the lines of apparent block had long durations and were fractionated, a characteristic that can also result from activation transverse to the myocardial fiber long axis. Therefore, we propose that the parallel orientation of the muscle bundles in the epicardial border zone is an important cause of ventricular tachycardia because activation transverse to myocardial fibers is sufficiently slow to permit the occurrence of reentry.