This study examined analgesic efficacy and adverse effects of buprenorphine and fentanyl for the postoperative pain relief by continuous epidural infusion. Fifty patients after upper or lower abdominal surgeries were assigned to two groups and buprenorphine and fentanyl were epidurally administered postoperatively. Buprenorphine (B) group received bolus injection of B 0.1mg + saline 8 ml and continuous infusion of B 0.8 mg+saline 92 ml (2 ml.h-1). Fentanyl group received bolus injection of F 0.1 mg+saline 6 ml and continuous infusion of F 0.6 mg+saline 84 ml (2 ml.h-1). There was no significant difference between the two groups in the analgesic efficacy, which became lower from 2 to 12 hours postoperatively. However, compared with buprenorphine group, the incidence of nausea or vomiting and dizziness was significantly less in the fentanyl group (11 vs. 4 cases and 7 vs. 1 cases). These results imply that the major site of action of epidurally administered fentanyl is the spinal cord. In contrast, analgesic effect of epidural buprenorphine appears to be enhanced by the supraspinal action. We conclude that fentanyl is superior to buprenorphine for postoperative pain relief by continuous epidural infusion.