BACKGROUND Addition of bicarbonate to local anesthetics improves the efficacy of epidural anesthesia. We evaluated whether the addition of bicarbonate to lidocaine enhanced pain threshold in cesarean section. We speculated that bicarbonate would increase the concentration of lidocaine in cerebrospinal fluid (CSF). To examine this possibility, we evaluated the relationship between the lidocaine concentration in the CSF and the pain threshold. METHODS Twenty eight full-term parturients undergoing cesarean section under epidural anesthesia at L2-3 were divided into two groups: the first group (lidocaine group) received 17 mL of 2% lidocaine-epinephrine (1:200,000) and the second group (lidocaine-bicarbonate group) received the same concentration of lidocaine-epinephrine supplemented bicarbonate. Twenty min after administration of local anesthetics, we collected 1 mL samples of the CSF. The pain threshold after the repeated electrical stimulation was used to assess sensory blockade at the L2, S1, and S3 dermatomes. RESULTS Demographic data were comparable between the groups. There were no differences in the pain threshold at all dermatomes and the lidocaine concentration between the groups. There was a significant correlation between the pain threshold and the lidocaine concentration at the combined S1 and S3 dermatomes in the lidocaine-bicarbonate group. CONCLUSIONS We find neither bicarbonate caused a significant difference in the efficacy of epidural anesthesia nor it caused an increase of lidocaine concentration in the CSF. The result that we found a significant correlation between the lidocaine concentration in the CSF and the pain threshold at the sacral region in the lidocaine bicarbonate group suggests that, bicarbonate enhances the efficacy of anesthesia at outside of spinal canal.