BACKGROUND: The maximum recommened dose for lidocaine and mepivacaine with epinephrine is about 500 mg regardless of the injection site. But this dose is insufficient for brachial plexus block and it is known that mepivacaine is safer than lidocaine. So the purpose of this study was to determine whether the serum concentrations of 1.5% lidocaine 600 mg with epinephrine and 1.5% mepivacaine 600 mg with epinephrine are within a safe range and to know any differences between them during axillary brachial plexus block. METHODS: Fifteen ASA physical status class I-II patients were selected. They were randomly divided into two groups. Group L patients (n=8) were anesthetized with 40 ml 1.5% lidocaine (600 mg) with epinephrine and group M patients (n=7) with 40 ml 1.5% mepivacaine (600 mg) with epinephrine. The serum concentrations were measured at the 5, 15 and 30 minutes after injection. RESULTS: The serum concentrations were 1.99 +/- 1.47, 2.66 +/- 1.03 and 2.41 +/- 1.41 microgram/ml in group L, and 3.03 +/- 0.64, 4.07 +/- 0.93 and 3.99 +/- 1.02 microgram/ml in group M at the 5, 15 and 30 minutes after injection. There were no symptoms or signs of local anesthetic toxicity, and serum levels of lidocaine and mepivacaine remained below the level that usually caused toxic symptoms. CONCLUSIONS: Although the serum concentration of mepivacaine group was higher than that of lidocaine group at each time, 1.5% lidocaine 600 mg with epinephrine and 1.5% mepivacaine 600 mg with epinephrine can be safely administered into the axillary sheath because the serum concentrations are within a safe range.