Abstract Light and electron microscopy of single myelinated fibres have been used to study chronic entrapment of the median nerve at the wrist in guinea-pigs. In mildly affected nerves it was found that both proximal and distal to the site of entrapment the internodes were bulbous at one end and tapered at the other. On the proximal side of the lesion it was always the proximal ends of the internodes which were bulbous; distal to the lesion the polarity was reversed. In more severely affected nerves paranodal demyelination occurred at the tapered ends of the internodes. It is argued that the reversed polarity of the changes in the two sides of the lesion indicates a mechanical rather than an ischaemic cause for the nerve damage.