The influence of isoenzyme pattern of myosin on cardiac energetics was investigated in a modified in situ heart-lung preparation in the rat. Chronic pressure load (spontaneous hypertension, aortic stenosis, Goldblatt hypertension), intermittent feeding, and swim-training elicited redistribution in the concentration of alpha chains of myosin ranging from 18 to 94%. The influence of isoenzyme pattern of myosin on cardiac energetics could be quantitatively assessed by extrapolation of the regression line of oxygen and substrate consumption related to tension time index. Fast myocardium with 100% alpha chains had an ATP and oxygen consumption which exceeded that of slow myocardium with 0% alpha chains by about 60%. This corresponds well to the state of activity of myofibrillar ATPase of fast myocardium which also exceeds that of slow myocardium by about 50%. Furthermore it could be shown that acute increase in the ATPase activity depends on the isoenzyme pattern of myosin. Under the influence of catecholamines the oxygen consumption related to tension time index increased by 30-40% in fast myocardium, whereby in a myocardium with 40% alpha chains no increase in oxygen consumption per unit tension time index was observed, when catecholamines were applied.