【Abstract】Objective To evaluate the effects of different doses of dexmedetomidine compounded propofol and fentanil on hymodynamics and intraoperative somatosensory evoked potentials (SEPs)and motor evoked potentials(MEPs) monitoring on the senile people Methods Forty-five patients selectedly undergoing anterior spinal fusion (PSF) surgery were randomly divided into three groups by the dose of Dex: group C (normal saline ); group D1 (Dex injected by0.3 ug•kg-1•h-1）; group D2 (Dex injected by0.8 ug•kg-1•h-1）. The induction of all patients in three groups ：midazolam 2~3mg,propofol Cp 1.5 μg/ml(Marsh model), fentanil 1.5~2μg/kg .When the patient’s consciousness disappeared,a LMA was intubated and mechanical ventilation was made.In group D1 and group D2,dexmedetomidine (0.5 μg/kg over 10 min) was administered and then respectively, followed by an infusion of 0.3 μg•kg-1•h-1 or 0.8 μg•kg-1•h-1 until the end of surgery.In group C, the same dose of NS instead of dexmedetomidine was administered and other methods of drugs giving and anesthesia were the same in group D1andD2.All patients were administrated by propofol and fentenil to keep bispectral index(Bis) from 45~55 by adjusting the concentration of propofol target controlled injection(TCI).Muscle relaxant wasn't inputed in whole operation and SEPs and MEPs were meatured by certain people during the operation. Results A more conscious time was observed in those patients of group D2 than group D1（P<0.05）.There was a lower heart rate in the cohort of dexmedetomidine ,but no difference on mean arterial pressure(MAP) .About SEPs P15-N20 ,neither duration nor amplitude has a significant difference in three groups (P>0.05).Amplitude of MEPs in three patients of group D2 disappeared in different degrees.There was a significant difference from the cases of group D1 and C.After dexmedetomidine was stopped about 20 min-30 min, amplitudes of MEPs recoveried in all the three patients. Conclusions In senile patients, dexmedetomidine possibly prolonged analepsia time and decreased heart rate. Dexmedetomidine dose not significant influence the SEPs, but higher dose of dexmedetomidine depressed MEPs.