Ropivacaine has been introduced for use in epidural analgesia in labor. However, there have been few formal dose-response studies of ropivacaine in this setting. Methods: The authors performed a prospective, randomized, double-blind study examining the effectiveness of five different doses of ropivacaine (10, 20, 30, 40, and 50 mg) administered epidurally in a volume of 10 ml to establish analgesia in 66 parturients who were in active labor with cervical dilatation less than 4 cm. A dose was

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... sidered effective when the visual analog scale pain score decreased by 50% or more from baseline. Results: A sigmoid dose-response curve and a probit log dose-response plot (linear regression coefficient, r ‫؍‬ 0.84; coefficient of determination, r 2 ‫؍‬ 0.71) were obtained. The ED 50 (median effective dose) obtained based on the maximum likelihood estimation was 18.4 mg (95% confidence interval, 13.4 -25.4 mg). Time to onset of analgesia, duration of analgesia, time to two-segment regression of sensory block level, and incidence of motor block were not affected by the dosage of ropivacaine administered (P ‫؍‬ 0.93, 0.12, 0.55, and 0.39, respectively). However, the upper level of sensory block was doserelated (P < 0.01). Conclusion: In a traditional dose-response study, the ED 50 of ropivacaine required to initiate epidural analgesia in early labor was found to be 18.4 mg (95% confidence interval, 13.4 -25.4 mg).