Several recent reports have described the use of intrathecal hyperbaric and plain ropivacaine for obstetric and non-obstetric patients.In comparison with plain solution, hyperbaric ropivacaine produced faster onset and recovery, higher cephalic spread block height and a greater success rate of anesthesia1-2.Whether variation in injectate baricity may affect the potency ratio of ropivacaine remains to be substantiated.The aim of this prospective, randomized study was to determine the minimum effective local anesthetic dose (MLAD) of intrathecal hyperbaric and hypobaric ropivacaine for elective cesarean section and to define their potency ratio using the up-and-down sequential allocation method.