The conduction velocities of excitation of the skeletal muscle fibers of man and rat were measured by intracellular microelectrode techinigue, and the differences of conduction velocities and diameters of the muscle fibers were compared. Materials were the M. rectus abdominis of man and M. gastrscnemius and M. rectus abdominis of rat. 1. In the first case of man, the eonduction velocities of M. rectus abdominis were distributed between 2.59m/sec. 4.30m/sec. In the second case of man, that of M. rectus abdominis were distributed between 2.78m/sec and 4.31m/sec. In the rat, that of M. gastrocnemius were distributed bctween 0.94m/sec and 4.75m/sec. In the rat, that of M. rectus abdominis were distributed between 1.37 and 5.01m/sec. 2. There was no difference between diametars of the skeletal muscle fibers of man and that of rat. 3. It may be reasonably concluded that the difference of the conduction velocity between the muscle fiber of man and that of rat was not depend apon the differences of their diameters. 4. So it may be concluded that the difference of conduction velocity between man and rat was depend upon the difference of electrical charactor.