Naloxone, an opiate antagonist, has been reported to improve spinal cord blood flow and restore the neurological function of spinal cord in experimental animals. The present study investigated the short term effect of naloxone infusions in acute preparations. An impact of a 20-g weight dropped from 6 cm high was used to traumatize the thoracolumbar spinal cord in cats. Treatment was started immediately after trauma. The naloxone-treated cats received intrathecal (2 mg in 0.5 ml) as well as systemic (2-mg/kg bolus, 2 mg/kg/hour for 4 hours) doses of naloxone. In the control group, equal volumes of saline were administered. The physiological integrity of the spinal cord as measured by averaged spinal evoked potentials (SEPs) ceased immediately after trauma in both groups. No recovery of the SEP in response to sciatic nerve stimulation occurred in either group during the 4 hours of treatment and monitoring in this study.