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1. Dorsal root potentials (DRP) were recorded in the lumbosacral spinal cord of decerebrated unanaesthetized cats, following afferent volleys restricted to the thin myelinated (Group III) cutaneous afferents of the hind limb. The thick myelinated fibres (Group II) were blocked by a depolarizing current.

2. A pure Group III volley produced a DRP of negative polarity, called the III—DRP, signalling a depolarization of the intraspinal terminals of afferent fibres. The longer latency of the III—DRP when compared to that of the DRP after a Group II volley was accounted for quantitatively by the lower conduction velocity of the Group III fibres.

3. Special attention was given to the presence of III—DRPs having a predominantly positive polarity thus signalling a presynaptic hyperpolarization. Such `positive III—DRPs' were, however, never observed in this investigation.

4. Both after transection of the spinal cord at various levels and after administration of pentobarbitone the III—DRP persisted at normal polarity. The duration of the DRPs was increased by these experimental procedures.

5. The implications of these findings are discussed in relation to the prominent role postulated for the Group III fibres in the context of the gate control theory of pain.