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Although ketamine has been administered splnally in humans, previous neurotoxicity studies have shown that it can induce spinal cord lesions in various animal models. The aim of this work was to evaluate by histologic and blood-brain barrier studies whether different components of the commercial ketamine solution might be responsible for the microscopic lesions observed.Forty white New Zealand rabbits were randomly assigned to four groups of 10. One-percent preservative-free ketamine (0.3 ml), 1% d ketamine, 0.05% chlorobutanol, and 1% lidocaine were intrathecally injected through the atlantooccipital membrane. Laminectomy was performed on day 8, and the dura was preserved using paraformaldehyde-glutaraldehyde fixative. Light and fluorescence microscopy were performed on transverse spinal cord sections by a neuropathologist unaware of injected agents used. Specimens were then graded as normal or abnormal as compared with a control group receiving lidocaine.Isomers of ketamine did not induce spinal cord lesions in either study, but chlorobutanol (the preservative used in the ketamine solution) induced significant severe spinal cord lesions in both studies.The appearance of spinal cord lesions after intrathecal chlorobutanol strongly suggests that this preservative is responsible for apparent toxicity of ketamine and therefore should not be used in any solution intrathecally injected into humans.