We found persistent abnormalities in the recovery of membrane excitability in long-term regenerated motor nerve fibers in the cat as indicated in the companion paper. These abnormalities could partly be explained by membrane hyperpolarization. To further investigate this possibility, we compared the changes in excitability in control nerves and long term regenerated cat nerves (3-5 years after tibial nerve crush) during manoeuvres known to alter axonal membrane Na+/K+ pump function: polarization, cooling to 20°, reperfusion after 10 min ischemia, and up to 60 seconds of repetitive stimulation at 200Hz. The abnormalities in excitability of regenerated nerves were reduced by depolarization and cooling and increased by hyperpolarization and during post-ischemia. Moreover, the time course of recovery of excitability from repetitive stimulation and ischemia was prolonged in regenerated nerves. Our data are consistent with an increased demand for electrogenic Na+/K+ pumping in regenerated nerves leading to membrane hyperpolarization. Such persistent hyperpolarization may influence the ability of the axon to compensate for changes in membrane potential following normal repetitive activity.