This in vivo study demonstrates excellent stability of high concentrations of intrathecal bupivacaine and opioid mixtures. No nonreversible neurological complications were identified in patients receiving daily doses of bupivacaine up to 21.4 mg. Tolerability was variable because of motor weakness. Given that all intrathecal local anesthetics may be neurotoxic, caution must be exercised if high concentrations and daily doses are to be delivered over prolonged periods.