We studied the prolonged injury discharge in axotomized myelinated (A) and unmyelinated (C) nerve fibres within the first 15 min following sural nerve transection of the rat. Immediately after axotomy, 18% (22/122) of the C fibres generated action potentials, but none of 73 A fibres. This discharge ceased during the first 2 min after axotomy in most of the C fibres. The discharge frequency in the first 15 s after nerve cut varied between 0.3 and 7.2 Hz (mean 2 Hz) and decreased to a mean of 0.3 Hz 4 min after nerve lesion. C fibres displaying initial discharge frequencies of > or = 2 Hz ceased to generate action potentials significantly sooner than those with initial discharge frequencies of < 2 Hz.