Abstract: Objective:To determine the minimal motor-blocking concentration (EC₅₀) of epidural ropivacaine and bupivacaine in the postoperative epidural analgesia,and compare the relative potency of these drugs used in epidural space.Methods:Sixty ASA I women who went gynaecologic operation and requested epidural analgesia were randomized to receive either ropivacaine or bupivacaine.Epidural catheter was placed at L_2-3 and advanced for 3cm in the epidural space in a cephalad direction.Each received a 10 ml bolus of epidural ropivacaine or bupivacaine.Half an hour before and after the bolus,take a Bromage score to compare.Up-down sequential allocation was used to determine subsequent concentration:to increase the concentration one level if the previous woman did not respond and to reduce the concentration one level if the previous woman did respond.The up-down sequences were analyzed by using the Brownlee method to quantify the minimal motor-blocking local analgesic concentration.Results:The ropivacaine was 0.706% and for bupivacaine was 0.443% with a ropivacaine/bupivacaine potency ratio of 0.63 (95% CI,0.60-0.66).Conclusions:Ropivacaine was significantly less potent for motor block,at 63% that of bupivacaine.