Male Fischer 344 rats were subjected to regular forced exercise beginning at weaning and continued until sacrificed at ages ranging from 2 to 18 months. Littermates served as sedentary controls. At all ages examined, the exercised animals had greater heart and lower body weights. Actomyosin was isolated from the myocardium and the ATPase activity of the contractile protein preparation was measured. Only the sedentary animals exhibited an age-related loss in enzyme activity. The physically trained animals had substantially higher ATPase specific activity than the controls at all ages. This difference in ATPase specific activity may be the result of the synthesis of different isozymic forms of myosin or changes in the regulatory proteins of actomyosin (tropomyosin and troponin). It is suggested that lifelong regular exercise may alter the known biochemical changes in the heart muscle that relate to declining cardiac function.