Abstract： Objective To determine the effects of Dexmedetomidine postconditioning and hypoxic-ischemic time on brain damage in neonatal rats. Methods A total of 140 clean and healthy neonatal SD rats aged 7 days were randomly divided into 7 groups(n=20 each):the sham operation group(group S),hypoxia-ischemia group (group C),and Dexmedetomidine intervention groups at different hypoxic-ischemic time points(T1 group,T2 group,T3 group,T4 group and T5 group). The rats were weighed and anesthetized,and the left common carotid artery was fixed and ligated in the prone position,whereas group S was not ligated. Groups T1,T2,T3,T4 and T5 were injected with 100 μg/kg(4 μg/ml)Dexmedetomidine intraperitoneally immediately after ligation of the left common carotid artery,and at 3,6,12 and 24 hours after the ligation, respectively. The weight and mortality of the rats were recorded at the baseline and within 7 days after the model establishment. At 7th day after the model establishment,the brain tissues were taken,and the masses of the left and right cerebral hemispheres were weighed,and the mass ratio of the left and right cerebral hemispheres was calculated. The brain tissue sections were performed with Nissl's staining. The neurons at the posterior cingulate cortex and hippocampal CAI region were determined,and the density ratio of the right and left normal neurons was calculated. Results There was no statistically significant difference in the body weight among the groups(P>0.05). There was no death in group S,and there was no statistically significant difference in the mortality among groups C,T1,T2,T3,T4 and T5(P>0.05). Compared with group S,the ratio of left cerebral hemisphere mass and bilateral cerebral hemisphere mass in groups C,T1,T2,T3,T4 and T5 decreased,and the difference was statistically significant(P<0.05). Compared with group C,the ratio of left cerebral hemisphere mass and bilateral cerebral hemisphere mass,and the density ratio of normal neurons at the posterior cingulate cortex and hippocampal CAI region increased in T1,T2 and T3 groups(P<0.05). There were no statistically significant changes in the parameters between T4 and T5 groups(P>0.05). Compared with group T3,the ratio of left cerebral hemisphere mass and bilateral cerebral hemisphere mass, and the density ratio of normal neurons at the posterior cingulate cortex and hippocampal CAI region increased in T1 and T2 groups(P<0.05). There were no statistically significant changes in the parameters between T1 and T2 groups (P>0.05). Conclusion Dexmedetomidine post-conditioning in neonatal rats within 6 hours of ischemia and hypoxia can reduce brain damage,whereas there will be no significant cerebral protection over 12 hours.