Adult male rats were divided at random into two groups and trained on the treadmill running schedules, 60 min/day and 10min/day for group A and B separately. Four weeks later the ultrastructure of the left ventricular myocardium was examined and the specific surface of mitochondria wasmeasured. Comparing to the corresponding control group, the specific surface of mitochondria in the two groups of trained rats was decreased significantly. (5.883+0.12 um and 6.050?0.12 um to 6.564?0.17 um). This means myocardium mitochondria in trained rats were swelling. The reason might be that the myocardium injury was induced by exercise training.