BACKGROUND: Nitric oxide (NO) is known to play causative role in the development of neuropathic pain following peripheral nerve injury. However, it is yet to be investigated whether the role of NO differs in pain modalities, such as mechanical and thermal stimuli. Also, it has not been investigated whether NO has different roles in the stages of neuropathic pain - its development and maintenance. METHODS: Neuropathic pain was induced by a resection of the lumbar dorsal root 5, 6 (L 5, 6). After N-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor was injected intrathecally or locally around the dorsal root, we observed the behavioral response to the mechanical and thermal stimuli. RESULTS: Mechanical and thermal allodynia was inhibited by the application of L-NAME before the dorsal root injury. However, L-NAME did not affect the mechanical and thermal allodynia during the maintenance of neuropathic pain. CONCLUSIONS: We suggest that NO in the spinal cord or injured perineural site may play an important role in the induction of neuropathic pain, and may be associated with mechanical and thermal allodynia.