The changes of rat muscle fiber structure, fiber types and the distribution after the reunification of the nerve and cross-innervation between the nerve to M. soleus (SOL) and M. extensor digitorum longus (EDL) were cytologically studied and the following results were obtained: (1) In cross-innervation experiments, changes of the fiber types occured, namely, the white fibers distinctly appeared , in the SOL normally composed of the red and intermediate fibers, as if the SOL converted to EDL which was ordinarily consisted of three types of muscle fibers. This result was ascertained by an electron-microscopic study, and the white fibers, found in cross-innervated SOL, had the motor endplates and sarcoplasmic reticulum as in the original white muscle fibers. (2) In the reunification study of the nerve to SOL and EDL, the muscle fibers, fallen in denervation-atrophy stage, were regenerated to almost normal in constructure. The distribution of the muscle fibers, however, was changed; a tendency to be grouping a single fiber type was noticed in the muscle fibers. (3) These results suggested that the distribution of muscle fibers were greatly influenced by the peripheral nerve control.