Changes in the threshold for antidromic activation of C-fibre afferent terminals in the lumbar spinal cord of the decerebrate-spinalized rat have been examined. 2. The antidromic compound action potential elicited by stimulation in the dorsal horn was recorded in a sectioned dorsal root. 3. The antidromic C wave had conduction velocities and strength-duration properties similar to that described for other unmyelinated fibres. 4. The optimal position of the stimulating electrode within the spinal cord for eliciting the antidromic C wave was found to correlate with the site of entry and termination of C-afferent fibres. 5. Local stimulation within the dorsal grey of the spinal cord was shown to produce prolonged increased excitability of the C-afferent terminals in that segment. This effect was restricted to the terminals and could not be demonstrated in the stem axons of the C fibres. 6. Cutaneous afferent conditioning volleys from the sural nerve produced marked increases in the excitability of the C-afferent terminals. This effect was present at A-fibre strength sural stimulation, with no significant alteration when C-fibre strength stimulation was used. The alteration in the threshold for antidromic stimulation produced by the sural conditioning stimuli only occurred at the C-afferent terminals and not at their axons. 7. The results are discussed in terms of presynaptic inhibition of C-fibre input at a segmental level.
