Studies from this laboratory have shown that the border of a 24-hour canine infarct is histologically sharp and is composed of numerous interdigitating peninsulas of necrotic and normal tissue. To see if this sharp boundary is spatially related to the capillary beds of occluded and non- occluded arteries, the left anterior descending artery (LAD) was ligated in five mongrel dogs. Twenty- four hours later, white silicone rubber (Microfll) was injected into the LAD distal to the ligature; simultaneously and under the same pressure, red Microfil was injected into the left main coronary artery (LMCA). In hematoxylin and eosin sections from the border of the infarct, capillaries supplied by the LAD (white) were either in areas of necrosis, in normal epicardium or, rarely, in normal tissue along the lateral boundary; those supplied by the LMCA (red) were almost always in normal regions. Quantitative evaluation of this relationship revealed that the majority of the vessels in the normal and necrotic tissue were concordant (i.e., that normal tissue was supplied by the LMCA, and necrotic tissue by the LAD). However, a small zone of vascular discordance, averaging approximately 30 /im in width, was present along the infarct boundary, possibly representing a narrow border zone of little conse- quence. Hence, the complex interdigitation of normal and necrotic tissue in the lateral border of an infarct is predominantly a function of the interdigitation of the capillary beds supplied by the occluded and nonoccluded arteries.