The effect of long-term continuous subcutaneous infusion of naloxone on blunt spinal cord injury in the rat was assessed using four tests of neurological function, seven histological categories, and two electrophysiological measures. All four neurological function tests showed a trend toward improvement in naloxone-treated animals: the degree of improvement was statistically significant in two of the four categories. A significant reduction in myelin sheath edema was found in the naloxone-treated animals. Although there was a decrease in corticomotor-evoked potentials complexity following injury, there was no significant difference in naloxone-treated animals. Somatosensory-evoked potentials were significantly increased in amplitude and latency in naloxone-treated animals. This increase was most apparent at 60 min: no difference was found by 3 weeks postinjury. These results confirm earlier reports that naloxone can ameliorate the functional neurological deficits of spinal cord injury. Naloxone also produces alterations in the somatosensory-evoked responses in the early phase of treatment and significantly reduces myelin sheath edema.