Microelectrode recordings were used in conjunction with antidromic activation to monitor impulse conduction along individual mammalian cerebral axons for periods of up to 165 days. Approximately half of the axons studied showed a stable conduction velocity and stable aftereffects of impulse activity. The remaining axons showed slow and progressive increases or decreases in conduction velocity overtime. In these latter axons, changes in the magnitude of the aftereffects of impulse conduction were far less pronounced than were changes in axonal conduction velocity.