Effects of long-term with high intensity endurance training upon histochemically assessed myofibrillar actomyosin ATPase, myosin composition and creatine kinase content were analyzed in muscles from rats which were trained to run up to 240min/day at 40m/min. Following 16 weeks of the training, in M. soleus, proportions of slow-twitch oxidative (S0) fibers and slow type myosin light chains were increased; no changes in myosin light chain were significantly observed. Furthermore, a decrease in creatine kinase was found in this muscle, indicating that SO fibers might have acquired slower properties. On the other hand, in M. extensor digitorum longus, there was only a transformation between fast-twitch fiber subgroups, with a decrease in LC3f by which fast myosin constructed was characterized by the highest myofibrillar ATPase activities. These data suggest that fiber type transformation may be brought about not only from fast-twitch glycolytic (FG) fibers to fast-twitch oxdative glycolytic (FOG) fibers but also from FOG fibers to SO fibers by long-term with high intensity endurance training.