Chronic constriction injury (CCI) of the rat sciatic nerve, which within 3 days induces thermal and mechanical hyperalgesia and mechanical allodynia, is used as a model for pain resulting from nerve injury. Involvement of nerve growth factor (NGF) in the development of this hyperalgesia is suggested by the increase in the level of mRNA encoding NGF in cells in the injured area and in dorsal root ganglia at the level of the lesion and the greatly increased NGF levels (determined by ELISA) in the ganglia ipsilateral to the CCI. Application of anti-serum to NGF at the site of CCI delayed the appearance of hyperalgesia, whereas pre-immune serum appeared to enhance it. These results are consistent with the view that NGF is an important factor in the appearance of hyperalgesia associated with unilateral mononeuropathy.