In the chronically diseased ventricle, regions of myocardial fibrosis and cells with altered electrophysiologic properties provide the substrate for re-entry. Shown here are photomicrographs of canine epicardial cells. The panel on the left was obtain a week after acutely ligating a coronary artery to cause a myocardial infarction and shows parallel myocardial fibers, slightly separated by inflammatory cells and edema. The panel on the right, from an earlier work, shows a similar section two months after the infarction. There are now areas of fibrosis and disarranged but viable myocardial cells capable of resulting in marked electrical inhomogeniety, altered conduction pathways, regions of slowed conduction and unidirectional block.