Ropivacaine (LEA 103) is a new long-acting local anesthetic that in animal experiments has proved to be equally potent but less toxic than bupivacaine. In this controlled double-blind study in man, the dose-response relation of ropivacaine was examined in peripheral nerve block and its potency was compared to that of bupivacaine. Nerve functions during bilateral ulnar nerve block were monitored in 70 normal subjects by a computer-controlled method. Results indicate that plain ropivacaine is optimally effective at concentrations between 0.5% and 0.75%, where it reliably blocks nerve functions for about 8 hours. Its profile of action is uniform for the different fiber groups and resembles that of bupivacaine. Addition of epinephrine does not improve latency or duration of ropivacaine nerve block. Adverse effects attributable to ropivacaine have not been observed.