Antidromic action potential of large pyramidal neurons of rabbit''s motor cortex produced by stimulation of the medullary pyramid consisted mainly of a rapid surface-positive deflection and a slow surface-negative wave attributable to activity of axon and the apical dendrites, respectively. The dendritic component could be modified by cold within the limit between 20[degree] C and 38[degree] C, and could be abolished completely at temperatures below 20[degree] C. The axon apparently could endure the temperature change much better than dendrites. Anoxia and traumatic injury to cortical surface affected the dendritic component much more severely than the axonal component, especially in so far as the recovery process was concerned. Two main groups of fibers were identified in the pyramidal tract, the fastest of the first group conducting at a velocity of 30 m/second and the second group at 15 m/second. Conduction velocity of apical dendrites was estimated as being approximately 0.6 m/second in the proximal segment, 0.42 m/ second in the middle and 0.21 m/second in the terminal branches.