BACKGROUND: Whether the injured peripheral nerve in a late stage has repairing value still remains a problem. If irreversible changes happen in substance P and calcitonin gene-related peptide, the feeling function will lose even after repairing.OBJECTIVE: To quantitatively study the changes of substance P(SP) and calcitonin gene-related peptide(CGRP) in the spinal dorsal horn 24 weeks after peripheral nerve injury.DESIGN: A self-controlled quantitative experiment.SETTING: Institute of Orthopaedics of the Fourth Military Medical University of Chinese PLA.MATERIALS: This experiment was performed in the Institute of Orthopaedics of the Fourth Military Medical University of Chinese PLA from October 2002 to May 2003. Totally 55 SD rats were divided into 11 groups according to time points(1, 2, 3, 4, 6, 8, 10, 12, 16, 20 and 24 weeks after sciatic nerve transection).INTERVENTIONS: Sciatic nerve injury model was set up by transecting one side of sciatic nerve and ligating the proximal stump of sciatic nerve; the other side was set as the control side. Computer-assisted image analysis was used to measure the immunologic reaction areas of substance P and CGRP.MAIN OUTCOME MEASURES: Changes of the distribution of the positive fibres of SP and CGRP in rat spinal dorsal horn in each group.RESULTS: Fifty-five rats entered the result analysis. The distributions of SP immunoreactivity in the spinal dorsal horn following sciatic nerve injury showed a significant reduction during 2-6 weeks, followed by a slow rate of increase,and reached almost complete restoration at 16 weeks after deafferentation. No obvious advanced changes happened at 20 and 24 weeks. The ratios for ipsilateral and cotralateral sides of positive fibre and distribution area injury in spinal dorsal horn CGRP were 1.14 at week 1, 1.13 at week 6, and 0. 29 at week 24. The ratios were similar at each time point( P ＞ 0. 05).CONCLUSION: At the late stage of peripheral nerve injury, neurons in the spinal dorsal horn and dorsal root ganglion still remain their functions to synthesize and secrete SP and CGRP. Spinal dorsal horn remains at a balanced status and still has the neurologic basis to recover the sensory function.