Atrophy of rat muscle fibers by a tenotomy of the achilles tendon and a gips-fixation of the whole hind limbs were histochemically studied, and the following results were obtained. (1) Gips-fixation and tenotomy experiments, the red and intermediate muscle fibers were markedly fallen in atrophic stages. (2) After the tenotomy, denervated red and intermediate muscle fibers were scattered partially in the M. soleus. (3) By the tenotomy, the white muscle fibers kept almost normal in comparison with the red ones.(4) The atrophy of the muscle fiberss was more remarkable in the gips-fixation than in the tenotomy. This tendency was more marked in the M. soleus than the other muscles. (5) Mitochondria were more destroyed in quality than in quantity in atrophied muscle fibers.