In decerebrated spinal cats, stimulation of cutaneous Aδ and C afferents and intraarterial injections of bradykinin into a hind limb produced a reduction in the threshold for antidromic activation in the substantia gelatinosa of single C fibres. Meperidine increased the unconditioned threshold but potentiated the decrease in the threshold produced by Aδ fibre stimulation and by bradykinin injection. These results support the suggestion that myelinated cutaneous afferents can depolarize C afferent terminals. The observations also indicate that the depolarization of C afferent terminals produced during pain and by stimulation of other cutaneous afferents is potentiated by meperidine.