Abstract: Objective To study the effect of combined methylprednisolone and 21-aminosteroid tirilazad on blood flow in spinal cord,and somatosensory evoked potentials(SEP),histopathological changes and motor function of spinal cord. Methods Sixty canines were randomly divided into A1,B1,C1,A2,B2,C2 groups(10 in each group).A canine spinal cord injury model was established by depressing T13 with a water bag under SEP monitoring.Animals in A1,B1 and C1 groups were infused with methylprednisolone(30mg/kg iv for 15min) within 3h after decompression.After 45min,animals in A1 and B1 groups were infused with methylprednisolone(5.4mg/kg·h) for 23h and 47h respectively,while those in C1 group were infused with 21-aminosteroid tirilazad(1.5mg/kg·6h) for 47h.Animals in A2,B2 and C2 groups underwent the same treatment within 3-8h after decompression.SEP and blood flow in spinal cord were measured at 4 and 12h and on days 1,7,14 and 28 after decompression and scored with the modified Tarlov method.The injury range was assessed by histological analysis 28 days after decompression. Results No difference was observed in SEP,blood flow in spinal cord and motor function recovery of spinal cord in A1,B1 and C1 groups.However,the incidence of complications,such as infection,septicomyemia and pneumonia,was significantly higher in B1 group than in A1 and C1 groups,while the recovery of SEP was much better in B2 and C2 groups than in A2 group(P<0.05),the blood flow in spinal cord was remarkably greater and the motor function of spinal cord recovered noticeably faster in B2 and C2 groups than in A2 group(P<0.05),and the injury range of spinal cord was significantly less in B2 and C2 groups than in A2 group(P<0.05).The pathological change was associated with the SEP,blood flow in spinal cord,motor function recovery of spinal cord. Conclusion The motor function recovery of spinal cord is significantly different within 3h and 3h after drug treatment.Prolonged methylprednisolone treatment is significantly effective for the motor function recovery of spinal cord,but has more side effects.21-aminosteroid tirilazed can be used in treatment of spinal cord injury instead of methylprednisolone.