We made simultaneous recordings from 48 bipolar electrodes to obtain epicardial isochronal maps during induced ventricular arrhythmias in dogs 3-5 days after ligation of the left anterior descending artery. There was strong evidence of a reentrant circuit (RC) that was discernible largely on the epicardial surface in 21% of ectopic beats that were analyzed. Epicardial recordings were of limited value in analyzing the arrhythmia in other beats. The reentrant circuit consisted of an arc of conduction block around which the activation front advanced in a circular fashion at slow but uneven speeds while the rest of the ventricle was activated by radial spread. The arc of conduction block around which the RC was formed was usually functional in nature, since the same area of myocardium was excitable at relatively long cardiac cycle lengths. Both the length of the arc of conduction block which defines the length of the reentrant circuit and the degree of conduction delay were crucial factors for the occurrence of reentry. A premature beat that initiated reentry resulted in a longer arc of conduction block and slower conduction compared to one that failed to induce reentry.