BACKGROUND: Caudal injection of local anesthetics with morphine is the most common anesthetic technique for perianal operation and postoperative analgesia. This study was purposed to compare the onset time of caudal analgesia, postoperative analgesic effect and side effects. METHOD: Sixty healthy patients scheduled for perianal operation were divided into 2 groups randomly. Group I was given 2 mg of morphine in 20 ml of 2% mepivacaine via sacral hiatus. Group II was also given 2 mg of morphine in 20 ml of 0.5% bupivacaine caudally. We measured the onset time of caudal block, time to the first request of analgesics, the number of analgesics within 24 hours and the incidence of postoperative side effects. Analgesic effect was evaluated by visual analogue scales (VAS) at 1, 2, 6, 12 and 24 hours postoperatively. RESULT : The onset time of caudal block for operation and the first request time of analgesic for postoperative pain was significantly shorter in group I than group II. The analgesic use in the first 24 hours was significantly more in group I than group II. The side effects were similar in both groups. CONCLUSION: We concluded that the combined use of morphine and bupivacaine provided better postoperative analgesia than the combined use of morphine and mepivacaine.