Myelinated nerve fibres with a reduced safety factor for conduction due to demyelination are easily blocked by trains of impulses. To find out why, in vivo recordings from rat ventral root fibres demyelinated with diphtheria toxin have been supplemented with in vivo and in vitro recordings from normal fibres. Despite a small rise in extracellular potassium activity, normal fibres were invariably hyperpolarized by intermittent trains of impulses. This hyperpolarization resulted in an increase in threshold and also in an enhancement of the depolarizing after-potential and the superexcitable period. Replacement of NaCl in the extracellular solution by LiCl completely blocked both the membrane hyperpolarization and the threshold increase which were normally observed during intermittent trains of impulses. At demyelinated nodes which were blocked by trains of impulses (10-50 Hz), conduction block was preceded by a rise in threshold current and in an increase in internodal conduction time, but by no detectable reduction in the outward current generated by the preceding node. It was found possible to prevent the threshold from changing during a train by automatic adjustment of a d.c. polarizing current. This 'threshold clamp' prevented the conduction failure and virtually abolished the changes in internodal conduction time. The threshold changes were attributed to hyperpolarization, as in normal fibres, since (a) the polarizing current required to prevent them was always a depolarizing current, and (b) they were accompanied by an increase in superexcitability. The post-tetanic depression that can follow continuous trains of impulses was attributed to the combination of increased threshold and enhanced superexcitable period due to hyperpolarization. It is concluded that the susceptibility of these demyelinated fibres to impulse trains is not due to a membrane depolarization induced by extracellular potassium accumulation but to a membrane hyperpolarization as a consequence of electrogenic sodium pumping.