Abstract Whole nerve and single axon recordings were obtained in vitro from rat sciatic nerve demyelinated with lysophosphatidyl choline. Compound action potentials recorded across the lesion site were attenuated in amplitude, and showed conduction slowing and temporal dispersion. Application of the potassium channel blocker 4-aminopyridine (4-AP), markedly enhanced the response recorded through the lesion demonstrating both an increase in amplitude and duration of the response. From single axon recordings, we observed two patterns of change in spike waveform following application of 4-AP: broadening of single axon spikes, and bursting of single axon spikes. Some axons that displayed conduction block along demyelinated segments before 4-AP was applied, showed restoration of conduction.