【Abstract】 Objective To observe the effect of epidural injection with different dose of butorphanol on the rats’ serum myelin basic protein(MBP) and spinal cord morphology. Methods A PE-530 catheter was inserted into the epidural space of each Sprague-Dawley rat (male,weighting 180-210g) at L1-2 level，and three days later,a total of 32 rats without any motor dysfunction were randomly divided into 4 groups as follow:saline group (group NS,n=8)and butorphanol group( B1,n=8;B2,n=8;B3,n=8).Rats in group NS were epidurally injected NS 30μl each ,and rats in group B1,B2 and B3 were respectively epidurally injected butorphanol 60μg,120ug and 240μg(all diluted to 30μl with NS)1 time per day 5 days, the serum MBP of rats were measured before injection (T0),6h(T1),24h(T2) and 72h (T3) after the last injection by ELISA examination.At T3,the lumbar enlargement of the spinal cord,was taken for detecting the morphology and apoptosis with optical microscope and electron microscope. Results There was no significant difference between each group in content of the serum MBP (P>0.05).In group B2 and B3,compared with group NS and B1,the neurons showed degeneration and necrosis,the nissl body disappeared and the expression of the Bcl-2 decreased obviously while the Bax increased.Besides that,in group B2 and B3,the number of apoptosis neurons increased obviously (P<0.05) and the number of myelins diminished,while the structure of myelins was destroyed. Conclusion Repeated epidural injection of butorphanol 60ug has no influence on spinal cord morphology of rats,while injection of 120ug and 240ug can impair the spinal cord morphology.