A reduction in muscle blood flow caused by central vascular or peripheral vascular disease, or both, invariably leads to a reduction in exercise tolerance. Exercise conditioning programs have been shown to be an effective means of improving exercise tolerance in many patients with cardiovascular disease. However, improvements can be found without detectable changes in total muscle blood flow during activity. This suggests that peripheral adaptations induced within active muscles by exercise training exert a beneficial influence. These adaptations could include (1) a redistribution of blood flow within the active muscle or limb, or both; (2) an enhanced capillary density within the active muscle; and (3) an increased mitochondrial content within the active muscle. Evaluation of these adaptations, in an animal model of peripheral arterial insufficiency, suggests that all 3 factors potentially serve to enhance exercise tolerance after training. The improved ability of trained muscle to accommodate the metabolic stress during contractions is noteworthy. These factors could contribute to the marked increase in endurance during mild submaximal exercise observed in patients after training.