Intra-cellular potential changes of primary afferent fibers in the dorsal column of a cat''s spinal cord were recorded and analyzed. The slow potential change which has the same characteristics as that found in primary afferent fibers in the dorsal column of a frog''s spinal cord was observed in the intracellular potential change obtained from primary skin afferent fibers. The intracellular potential change of primary muscle afferent fibers of low threshold did not exhibit this kind of slow potential. The evoked spike potential of a muscle afferent fiber of low threshold is followed by a slight negativity and sometimes a long-lasting small positive after-potential. An analysis of intracellular potential changes was made to determine the after-potential of skin afferent fibers. A marked negative after-potential and also a long-lasting positive after-potential followed the evoked spike potential of these nerve fibers. Characteristics of the slow potential were analyzed and it is concluded that some kind of active depolarization of terminal endings of primary skin afferents give rise to: (1) this slow potential, (2) positive deflection of the dorsal cord potential, and (3) DRP V (Lloyd''s terminology). The existence of prominent negative and positive after-potentials of terminal endings of primary skin afferent fibers was discussed on the basis of results found in this experiment. It was suggested that the marked after-potential recorded from fibers in the dorsal column is the electrotonic potential caused by the large after-potential of terminal endings of these fibers.