To investigate possible peripheral mechanisms for post-injury sensory disorders in the trigeminal system, we have made electrophysiological recordings from myelinated fibres in the inferior alveolar nerve (IAN) which have previously sustained an injury. In earlier experiments we have shown that axons in ligature-induced neuromas of the IAN develop spontaneous activity and mechanical sensitivity. The present study has investigated these responses after two different types of injury. In 24 anaesthetised adult male ferrets the left IAN was either chronically constricted by four loose chromic gut ligatures (12 animals) or sectioned and regeneration permitted (12 animals). After recovery periods of 3 days, 1, 3, 6, 12 or 24 weeks, single unit recordings were made from the nerve proximal to the injury site. The proportion of units which were spontaneously active ranged from 0% to 19% after constriction injury and from 0% to 10% after nerve section and regeneration. Both groups revealed a marked variability between individual animals at similar time periods. Mechanical sensitivity was found in 0-42% of units after constriction and 0-25% of units after nerve section; both groups showed a significant negative correlation between mechanical sensitivity and recovery period. None of the fibres which had regained peripheral receptive fields was either spontaneously active or mechanically sensitive. There was no significant difference between the levels of spontaneous activity or mechanical sensitivity in the two groups or that previously found in ligature-induced neuromas. Thus we conclude that widely differing types of peripheral nerve injury are capable of initiating similar raised levels of afferent activity in myelinated inferior alveolar nerve fibres.