Muscle fibers in the transition zone of an infarcted heart are thought to be potentially ischemic during the first 6-9 h following coronary artery occlusion. However, the long-term fate of the muscle fibers at the margin of the necrosis is uncertain. Ischemia implies reduced oxygenation, possibly owing to a reduced capillary supply; thus our objective was to determine whether a region of microvascular supply exists at the margin of a necrosis produced by chronic coronary artery occlusion. Five variables were used to quantitate the capillary supply in the transition zone: C/F (capillary to fiber) ratio, V(f) (number of vessels around a fiber), F(v) (number of fibers surrounding a vessel), capillary density, and fiber diameter. Infarcts were induced in young males rats by ligating the left coronary artery midway between its origin and the apex of the heart. Five weeks later, the capillary supply in the transition zone was significantly below control values, i.e., significant reductions in C/F, V(f), and F(v) were found. This region of reduced capillary supply extended 225-525 μm laterally from the edge of the necrosis. Thus, a narrow transition zone, defined as a region of variable muscle fibers with a subnormal microvascular supply, exists as long as 5 weeks following coronary artery occlusion in the rat.