Objective: To compare the efficacy and safety of meperidine hydrochloride and valethamate bromide against placebo in shortening the duration of active labor. Method: We randomly assigned 160 nulliparous women with a singleton pregnancy at term who needed induction of labor to one of 3 treatments: 50 mg of meperidine (n = 53), 16 mg of valethamate bromide (n = 53), or a normal saline solution as placebo (n = 54). All medications were given by slow intravenous infusion. Labor duration was the main outcome measure. Results: The intervals between infusion and complete cervical dilation and between infusion and delivery were significantly reduced (P<0.001 and P<0.01) in the meperidine group (103.0 +/- 64.5 minutes and 119.8 +/- 70.2 minutes), in contrast to the placebo group (173.9 +/- 74.8 minutes and 192.2 +/- 82.8 minutes). However, differences were not significant between the 2 treatment groups (139.6 +/- 63.1 minutes and 160.6 +/- 71.9 minutes), or between the valethamate bromide and the placebo group. Conclusion: Meperidine, but not valethamate bromide, significantly shortened the duration of active labor in nulliparous women with a singleton pregnancy at term. (C) 2009 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.