Efficacy of continuous intravenous infusion for postoperative analgesia was evaluated in 20 patients who had undergone abdominal surgery for 72 hours postoperatively. The patients were randomly allocated to two groups: one group received continuous intravenous infusion of buprenorphine at 0.2 mg.24 hr-1 using a non-electronic, portable infusor 0.5 ml.hr-1 type (Baxter), while the other received infusion of pethidine at 50 mg.hr-1. During the first 12 postoperative hours, the frequency of "Fair" rating which indicated need of supplemental analgetics was significantly higher than the later 12-hour period until 72 hours in both groups. However, although during the first 12 hours continuous intravenous infusion was inadequate to alleviate postoperative pain compared with other 12-hour period, no patient received supplemental analgesics more than 2 times. During the 24 to 72 postoperative hours, 70% of cases needed no supplemental analgesics to alleviate postoperative pain. Continuous intravenous infusion of 0.2 mg.hr-1 buprenorphine or 50 mg.hr-1 pethidine was inadequate for postoperative analgesia during immediately after the operation to 36 hours postoperatively, especially during the first 12-hour period. However, this infusion was effective during 36 to 72 postoperative hours. There was no significant difference between buprenorphine group and pethidine group.