Objective To observe the dynamic changes of complement protein 3 in rats with the chronic constriction injury (CCI) of the sciatic nerve and investigate the relationship between spinal complement activity and hyperalgesia.   Methods Forty healthy male Sprague-Dawley rats were randomly divided into sham-operation group

CCI group

saline group and CVF group. The left sciatic nerve was ligated loosely according to Wang Jin-bao’s method in all rats except those underwent sham operation. The rats of CVF group were intraperitoneally injected with cobra venom factor after CCI

while the rats of saline group with saline as control. Pain threshold by thermal stimulation was recorded and the level of complement protein 3 in serum and cerebrospinal fluid (CSF) were determined by the immunoturbidimetry preoperatively and everyday in postoperative seven days

the levels of complement 3 in spinal cord were determined on postoperative day 7.  Results Thermal hyperalgesia developed after sciatic nerve ligation in CCI group

but not in sham-operation group. Cobra venom factor but not saline relieved the hyperalgesia. The level of complement 3 in serum in sham group

CCI group and saline group showed no changes. The postoperative levels of complement 3 in CSF of CCI group and saline group was significantly higher than that before operation (P0.01). The postoperative level of complement protein 3 in serum and CSF in CVF group exhibited lower than that before operation. The level of complement protein 3 in spinal cord was higher obviously in CCI group and saline group than sham group on postoperative day 7 (P0.05)

lower in CVF group than CCI group. The thermal hyperalgesia was correlated with the level of complement in spinal cord but not with that in serum.  Conclusion The complement components in central nerve system are activated abnormally after peripheral nerve injury

and the dynamic changes of complements may have an intimate relation with the formation of hyperalgesia.