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The possible contribution of non-myelinated afferent fibres (C fibres) to the mechanisms of primary afferent depolarization (p.a.d.) in the spinal cord of the rat has been investigated. Dorsal root potentials (d.r.p.s.) were recorded in the lumbar cord of normal adult rats, of adult rats which had been injected at birth with a solution of capsaicin (50 mgkg-1 s.c.) and of adult rats which had been injected at birth with the drug vehicle only. D.r.p.s were recorded from the dorsal rootlet that entered the spinal cord in the main area of termination of the tibial nerve. The location of this area was assessed by mapping the spinal cord in the rostro-caudal axis while recording cord dorsum potentials evoked by A-fibre volleys from the tibial nerve. No differences in peak amplitude, area or time to peak amplitude were observed between the d.r.p.s evoked in control and capsaicin-treated rats by stimulation of the tibial, sural or common peroneal nerves. The relation between the size of incoming A volleys to the spinal cord and the size of the d.r.p.s evoked by them was unaffected by the neonatal capsaicin treatment. Rats treated at birth with capsaicin showed a virtual absence of afferent C fibres as assessed from the lack of C waves in the compound action potentials evoked in each of the three nerves studied after antidromic stimulation of the dorsal roots. The presence of p.a.d. in control and in capsaicin-treated animals was also established using the technique of excitability testing of primary afferent fibres (Wall, 1958). No differences were observed between the p.a.d. recorded in control and in capsaicin-treated animals using this technique. D.r.p.s and p.a.d. (assessed by excitability testing of primary afferent fibres) were of similar magnitude in control and in capsaicin-treated rats anaesthetized with either sodium pentobarbitone or urethane. It is concluded that p.a.d. of myelinated afferent fibres produced by incoming volleys in myelinated afferent fibres is not affected by a life-long loss of non-myelinated afferent fibres.