A border or transition zone, comprising muscle fibers supplied by fewer capillaries than normal, and characterized by an altered capillary-fiber geometry, is present as long as 5 weeks following coronary artery occlusion in the rat. This zone, which extends 225-525 μm laterally from the margin of the necrosis, may be at risk of becoming hypoxic, especially under conditions of increased oxygen demand. Our objective was to determine whether exercise can stimulate capillary growth in this transition zone. Myocardial infarcts were induced by rats by ligating the left coronary artery, midway between its origin and the apex of the heart. After a 1-week recovery period, the animals underwent 4 weeks of voluntary exercise, 2 h/day, 6 days/week. Rats which had run a total distance of 5-10 km during the month had both a normal number of capillaries and normal capillary-fiber geometry in the transition zone. In contrast, rats that ran either less than 5 km or more than 10 km during the 4 weeks showed no significant improvement in the number of capillaries of capillary-fiber geometry within the border zone. Thus, under certain conditions, exercise can promote revascularization in the transition zone of infarcted rat hearts.