Focal non-freezing injury to rat sciatic nerve resulted in nerve conduction block and cessation of axoplasmic transport. Rats showed early functional recovery but subsequently developed a slowly progressive sciatic nerve paralysis. Horseradish peroxidase studies revealed prominent nerve oedema with early enhanced pinocytosis and later, passive leakage through damaged endoneurial capillaries. Detailed microscopy indicated a striking selective vulnerability to cold based on nerve fibre diameter. Unmyelinated fibres were spared while large myelinated nerve fibres showed severe axonal degeneration. Paranodal and segmental demyelination were infrequent findings and may have resulted from early axonal swelling. Possible pathogenic mechanisms to explain the degeneration of myelinated fibres in hypothermic injury are discussed. It is concluded that the gradient of clinical severity seen in limb hypothermic injuries has a pathological correlate in peripheral nerve.