The authors developed a new method of intrathecal local anesthetic injection in rabbits in order to study the relationship between anesthetic concentration and impaired neurologic function. They found that none of the local anesthetics studied produced persistent neurologic damage in concentrations used clinically. However, lidocaine and tetracaine can be prepared in high concentrations (far exceeding those clinically used) that will produce extensive irreversible neurologic injury and histologic changes. This was also true for sodium bisulfite, an antioxidant used in a number of commercially prepared local anesthetic solutions. Pure solutions of relatively insoluble local anesthetics (bupivacaine and 2-chloroprocaine) failed to produce comparable neurologic or neuropathologic changes when tested at concentrations up to their solubility limits. Extensive neurologic impairment was not necessarily accompanied by equally extensive lesions in the spinal cord and nerve roots.