Myocardial blood flow is the major determinant of oxygen delivery to the myocardium, since oxygen extraction by the myocardium is near maximum in the resting state. Regulation of flow during exercise depends on local metabolic factors and, to a small extent, on autonomic tone. Maximum flow of 5-6 times resting has been measured in reactive hyperemia experiments. In strenuous exercise, myocardial oxygen delivery appears to be adequate and flow reserve seems capable of handling the increased oxygen demand. No evidence of myocardial failure in normal hearts due to excess exercise has been presented. However, pulmonary hemorrhages found in horses after strenuous racing may be due to inadequate cardiac performance at maximal capacity. In humans, severe limitations to myocardial blood flow are imposed by coronary artery disease and by cardiac hypertrophy. In both cases regional myocardial ischemia may occur during the increased oxygen demands imposed by strenuous exercise. Individuals with coronary disease or cardiac hypertrophy are at risk for myocardial ischemia during exercise. Detection of myocardial blood flow abnormalities and ischemia during exercise has become an important goal in cardiac diagnosis. Prevention of serious or lethal consequence of strenuous exercise depends on a better understanding of the factors that regulate myocardial blood flow during ischemia.