The aim of this study was to determine the effects of sympathectomy on our previously developed animal model for neuropathic pain. The neuropathy was produced by a unilateral tight ligation of the L5 and L6 spinal nerves in 81 rats, all of which showed a marked increase in frequency of paw lifting in response to innocuous mechanical stimuli and a shortened latency of paw withdrawal in response to noxious radiant heat stimuli on the affected limb. We interpreted these as behavioral signs of mechanical allodynia and heat hyperalgesia. Surgical sympathectomy was performed by removing the sympathetic chain bilaterally from the L2 to L6 levels at 1 week prior to and 1, 3 and 5 weeks after nerve injury. In addition, the effect of sympathetic block was tested by systemically injecting guanethidine or phentolamine. Surgical sympathectomy relieved the signs of both mechanical allodynia and heat hyperalgesia. The effect of sympathectomy for mechanical allodynia is estimated to be almost fully expressed within 30 min after the operation. Sympathetic block by chemical agents reversibly relieved the mechanical allodynia. These data suggest that the rats in our model exhibit behavioral signs of neuropathic pain that are sympathetically maintained.