Abstract: Objective To observe the behavior and the changes of neurologic and his-topathologic structure of rats injected lidocaine-bupivacaine mixtures in different ratio.Methods Af-ter successful intrathecal catherization and positive lidocaine test,48 rats were randomly allocated into 6 groups with 8 rats in each group.Rats received intrathecally NS (group S),mixture of 2.5% lido-caine and 1.875% bupivacaine (group LB13),3.33%lidocaine and 1.67% bupivacaine (group LB12),5% lidocaine and 1.25% bupivacaine (group LB11),6.67% lidocaine and 0.83% bupiv-acaine (group LB21),or 7.5% lidocaine and 0.625% bupivacaine (group LB31).TFL and PWT on consecutive 4 days,recovery time of lower limbs and the pathologic changes in the spinal cord by method of microscopy and electron scope were observed by a blind investigator.Results TFL in group LB31was significantly longer(P <0.05)and hind legs motor function and recover time pro-longed obviously in group LB31;Edema vacuolar degeneration and interstitial edema in white matter of posterior horn of spinal cord were found in group LB31.The histopathologic score was obviously higher in group LB31 compared with other groups (P <0.05).On transmission electroscope,differ-ent levels of ultrastructural changes were found in all groups except in group S .Typical changes included interstitial edema ,loosening of the laminae of myelinated fibers , axon swelling and local degeneration with most serious injury in group LB31.Conclusion The spi-nal injury following intrathecal local anesthetics mixtures was more serious with higher concentration of lidocaine in the mixture.Lidocaine concentration lower than 5% mixed with 1.25% bupivacaine intrathecally induces no obvious spinal neurotoxicity.