OBJECTIVE Commercially available bupivacaine is a racemic mixture of S (-)- and R(+)-enantiomers. Although the S(-)-enantiomers levobupivacaine and ropivacaine are less toxic to the cardiovascular and central nervous systems than bupivacaine, their relative efficacy has not been determined. This study directly compares the dose response of levobupivacaine, ropivacaine, and bupivacaine following epidural and intrathecal administration in the rat. METHODS The time course of change in tail-flick latency and qualitative motor function was studied in rats following epidural or intrathecal administration of 0.25-0.75% levobupivacaine, ropivacaine, or bupivacaine in blinded, randomized fashion. RESULTS Levobupivacaine and bupivacaine produced comparable and significantly enduring antinociceptive effects compared with ropivacaine at all test concentrations following both epidural and intrathecal administrations. Duration of motor block at lower local anesthetic concentrations (epidurally and intrathecally) was comparable with levobupivacaine and ropivacaine but significantly shorter than with bupivacaine. Epidural 0.75% levobupivacaine and bupivacaine showed more enduring motor block than ropivacaine. CONCLUSIONS Levobupivacaine, given epidurally or intrathecally, produces longer lasting antinociceptive action than ropivacaine at equivalent concentrations and similar motor blocking effect at lower concentrations in both epidural and intrathecal administrations. Levobupivacaine-induced prolongation of the tail-flick latency is comparable to that of bupivacaine, as is motor blocking effect at higher concentrations. The possibility of significant differential block with levobupivacaine compared with bupivacaine warrants further study.