The spinal cords of 18 anesthetized cats were injured by standardized contusion. The animals were maintained for 4-16 months, then the thoracic spinal cord was isolated in vitro at 25 degrees C. Microelectrode recordings were made from single axons conducting through the lesion in ventral and lateral tracts. On warming the tissue, action potential conduction was found to block at temperatures below 36 degrees C in 29% of 129 axons tested. Of 17 axons in which it was possible to demonstrate a block below physiological temperature, apply 0.1-1 mM 4-AP, wait for 10 min and test conduction again, 7 showed increases in blocking temperature and 4 of these restored to conduction above 37 degrees C. The other 10 fibers showed no improvement in blocking temperature. 4-AP also increased the spontaneous activity of axons. It was concluded that 4-AP may eventually be useful in chronic spinal cord injuries, because it improves safety factor in some axons and increases excitability in others, which may compensate to some extent for the reduction in density of projections through the lesion.