BACKGROUND: This study was designed to determine the optimal dose range of epidural naloxone that can preserve analgesia while minimizing nausea, one of the most common side effects caused by epidural morphine. METHODS: Seventy-four patients undergoing combined epidural and general anesthesia for hysterectomy were randomly assigned to one of three groups. All received 2 mg epidural morphine bolus just before closing abdominal cavity and a continuous epidural infusion was started containing 4 mg morphine in 100 ml bupivacaine 0.125% with either no naloxone (Group 1, n = 24), 0.167 mg/kg/hr of naloxone (Group 2, n = 19) or 0.412 mg/kg/hr of naloxone (Group 3, n = 31) for postoperative pain control. Analgesia and nausea were evaluated by blinded observers. RESULTS: The combination of epidural morphine and bupivacaine provided good analgesia. Pain scores in group 3 were lower than in group 1 after surgery, but there were no significant statistical differences except at 16 hr. Group 2 showed the lowest pain scores at 8, 16 and 24 hr (P < 0.05). Nausea scores were lower in group 2 and 3 than in group 1 at 16 and 24 hr (P < 0.05). CONCLUSIONS: Epidural administration of naloxone below 0.412 mg/kg/hr was optimal and safe dose range that maintained the analgesic effects of morphine while minimizing nausea.