Some properties of late after-potentials which appear following a train of impulses were examined in frog skeletal muscle fiber. The decay of the late after-potential followed a simple exponential time course. The time constant of the decay was larger in a viscous solution than in normal Ringer solution. It was proved by physical experiments that the diffusion of K ions was delayed in the viscous medium at the same rate as the decay. The effect of temperature on the decay was low and the Q10 for the time constant was 1.2. When the late after-potentials were recorded at membrane potentials variedly controlled by the polarizing current, the reversal potential shifted in the positive direction with the increase of impulses. These results suggest that the late after-potential may be dependent on K ions accumulated in the T system. During the initial 300 msec period immediately after the onset of the decay, the amplitude was smaller than expected by a simple exponential time course. This effect was especially apparent in the sucrose hypertonic Ringer solution in which the decay was extremely extended. The cause of this non-exponential component was discussed with respect to the K accumulation hypothesis.