Six groups of ten women each in active labor at term had epidural catheters placed in the usual manner and received a 3 mL test dose of 2% lidocaine with epinephrine. Groups 1-6 received, respectively, 5, 10, 20, 30, 40 and 50 micrograms of sufentanil diluted to 10 mL with normal saline. Significantly effective analgesia was provided at all sufentanil doses studied, with pain scores decreasing from 8.1 +/- 0.2 at baseline to 2.9 +/- 0.3 at 10 minutes and 1.1 +/- 0.2 at 30 minutes (mean +/- SEM, average for all groups). The duration of analgesia showed a significant (p less than 0.05) relation to sufentanil dose, increasing from 79.1 +/- 11.3 minutes (5-micrograms group) to 137.8 +/- 17.2 minutes (50-micrograms group). There were no serious maternal side effects, although ten patients developed pruritus, four became dizzy, two experienced mild sedation, and one had transient hypotension. No neonatal side effects occurred. Maternal serum sufentanil levels remained below the sensitivity of the assay, or 0.1 ng/ml.