It has been suggested that low-threshold sensory pathways have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. In the present study, we examined the involvement of these pathways in the development of hyperexcitability after sciatic nerve injury (SNI) by detecting the intracellular signal molecule. The rats that received a transection of the sciatic nerve 7 days before were electrically stimulated at 0.1 mA and 3 mA in the proximal region of the nerve injury site. We found a small number of phosphorylated extracellular signal-regulated kinase (pERK)-labelled neurons in laminae I-II and III-IV of the spinal dorsal horn in the control rats after 0.1 mA stimulation. By contrast, there was a marked increased of pERK-labelled neurons both in the superficial laminae and laminae III-IV after the same stimulation in the SNI rats. Enhancement of ERK activation induced by 3 mA stimulation was also observed. Immunoreactivity of pERK in gracile nucleus neurons was also dramatically increased after 0.1 mA stimulation to the injured nerve. These data suggest that the rats with peripheral nerve injury had an increased responsiveness to the low- or high-threshold peripheral stimuli in I-II, III-IV and gracile nucleus neurons. Furthermore, SNI rats that received neonatal capsaicin treatment showed a decreased number of pERK neurons after 0.1 mA stimulation in the dorsal horn and gracile nucleus neurons compared to the control rats. Thus, C-fibres may contribute to the enhanced excitability of the low-threshold sensory neurons after peripheral nerve injury.