We investigated the possibility of transformation from slow-twitch fibers into fast-twitch fibers following after the high-intensity interval training. M. soleus and M. gastrocnemius of Wistar male rats were used for histochemical analysis to determine fiber type distribution and two-dimensional electrophoresis to examine myosin light chain. After 16 weeks of the training, hypertrophy of FOG fibers in the peripheral portion of M. gastrocnemius was observed. However, fiber type distribution in M. gastrocnemius and M. soleus was not changed. On the other hand, in molecular level, there was no change in myosin light chain after the training. These results indicate that it is difficult to transform fiber types from slow-twitch into fast-twitch, and to Change isozyme distribution of myosin light Chain by the high-intensity interval training.