Exercise training has been reported to be beneficial in preventing and/or treating coronary heart disease. However, the primary underlying mechanisms and the relative importance of training induced changes in the coronary vascular bed are not clear at this time. The research outlined in this proposal will provide a better understanding of the effects of exercise training on basic coronary function by attaining 2 primary objectives: a. Determine the effects of chronic exercise training on the coronary microvascular control of myocardial transcapillary exchange. b. Characterize normal coronary capillary transport reserve and determine if chronic exercise training has an effect on the capillary transport reserve of the coronary vascular bed. Transcapillary exchange will be measured with the single injection indicator diffusion technique. Regional coronary blood flow and the heterogeneity of blood flow will be determined with the radiolabeled microsphere technique. The effects of myocardial oxygen comsumption, heart rate, left ventricular pressure, and neuro-humoral factors, and maximal vasodilation on myocardial transcapillary exchange will be determined in control and exercise trained animals.