Ngan Kee WD, et al. - This study was designed to describe the dose-response curves of bupivacaine, levobupivacaine, and ropivacaine (local anesthetics) when given intrathecally for labor analgesia. The intent was to determine values for D₅₀ (dose producing a 50% response) and to compare the calculated values of D₅₀ for levobupivacaine and ropivacaine with those for bupivacaine. Findings offered support to previous work demonstrating that compared with bupivacaine, intrathecal levobupivacaine and ropivacaine are less potent.