The changes in the conduction properties of peripheral nerve fibres of the mouse were examined during the phases of segmental demyelination, and remyelination, following the intraneural injection of lysophosphatidyl choline (LPC). In this lesion the sequence of the morphological changes was well synchronised in the different calibre fibres and this permitted a closer correlation to be made between the histological and physiological events than has been possible with other experimental models. Conduction in the nerve fibres was blocked during demyelination and for 1--2 days after the axons were completely free of myelin. In many fibres, however, conduction was later restored at about the time the demyelinated axon membrane became reassociated with debris-free Schwann cells, but prior to remyelination; at this time the fibres were unable to conduct closely spaced impulses and the conduction velocity was low. During the subsequent period of remyelination the ability of the fibres to conduct closely spaced impulses was fully restored and the conduction velocity progressively increased; the remyelinated internodes remained both thinner and shorter than normal. The physiological properties of the fibres were described in terms of their refractory periods of transmission (RPTs) as well as their conduction velocities; only the description in terms of the RPT correlated well with the histological events and with the degree of the behavioural deficit.