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Using immunohistochemical analysis, we investigated the time profile of neuronal nitric oxide synthase (nNOS) expression in the lumbar spinal cord up to day 28 after transection of the L₅ spinal root. On day 14 after injury, we also evaluated the effect of intrathecal application of 7-nitroindazole (7-NI), a selective nNOS inhibitor (8.15 μg in 5 μl), on thermal hyperalgesia. Our results indicated that nerve transection increased the intensity of nNOS-immunoreactivity in superficial and deep laminae of the dorsal horn within a late stage (days 7 to 28) of the neuropathy model used. Furthermore, 7-NI attenuated nerve injury-evoked thermal hypersensitivity on day 14 but did not reduce it between days 2 and 5 after transection. These data suggest that nNOS overexpression is more involved in the development than in the initiation of thermal hyperalgesia in L₅-transected rats.