This study was conducted to determine whether nitric oxide (NO) is involved in the maintenance of behavioral signs of neuropathic pain induced by tightly ligating the left L5 and L6 spinal nerves. Neuropathic rats showed behavioral signs representing mechanical allodynia, cold allodynia and cold-stress exacerbated ongoing pain. Mechanical allodynia was suppressed by Nomega-nitro-L-arginine methyl ester (L-NAME; 200, 100, 50, 10 microM/kg, i.p.), a nitric oxide synthase inhibitor, in a dose-dependent manner. Cold allodynia and cold-stress exacerbated ongoing pain was also attenuated by L-NAME. Neither Nomega-nitro-D-arginine methyl ester (D-NAME; 200 microM/kg) nor saline changed any of the neuropathic pain behaviors. These results suggested that NO plays an important role in the maintenance of the behavioral signs of neuropathic pain and is involved in common steps in the maintenance of the different modalities of pain such as mechanical allodynia and cold allodynia.