Previous studies in experimental animals have indicated that physical activity can increase the extent of the myocardial vasculature but the optimal age, intensity, and duration of exercise that cause such modifications is not yet clear. The purpose of this study was to determine the effects of varying exercise intensity and duration on the myocardial vasculature of both prepubescent and postpubescent rats. Prepubescent animals were trained for 3 or 6 weeks at either a high or low exercise intensity beginning at 3 weeks of age. Postpubescent animals were trained in a similar manner beginning at 14 weeks of age. The high intensity exercise program administered prepubertally resulted in a significant increase in both the capillary density (25%) and the capillary to fibre ratio. The low intensity exercise program did not cause any modifications of the myocardial capillary density in the pre-pubescent animals. Postpubertally the high intensity 3 week exercise program caused a significant increase in the capillary density but no change in the capillary to fibre ratio. The low intensity exercise program did result in a significant degree of myocardial hypertrophy with a concomitant increase in capillary proliferation sufficient to maintain a constant capillary density. The results indicate that there may be a critical age related range of exercise intensity that is conducive to proliferation of the myocardial capillaries.