The effect of 18 wk of treadmill running on skeletal muscle metabolism and myocardial function of normal and myopathic hamsters was examined. BIO 14.6 hamsters could tolerate an exercise intensity of about 18 m/min for 40 min, 5 days/wk. Further increases in speed or number of bouts per day resulted in a falloff in performance. Normal hamsters could tolerate higher speeds and longer exercise bouts. Exercise did not change the severity of lesions of either the heart or skeletal muscle of the myopathic hamsters. A training effect was evidenced by increased succinate dehydrogenase activity in the soleus muscle. Cardiac function was evaluated as contractility measured from left ventricular pressure curves and expressed as (dP/dt)/kP. The results suggested that cardiac contractility was not as severely depressed in the trained BIO 14.6 strain of hamsters as in nontrained controls. However, (dP/dt)/kP was lower in the trained myopathic animals than in normal hamsters. ATP, CP, and glycogen levels were lower in myopathic hamsters with the lowest values occurring in the trained group. These data demonstrate that the BIO 14.6 strain of hamster can tolerate exercise training and that such training may have a positive effect on cardiac function.