Objective To study the changes of permeability of blood brain barrier (BBB) in rats after acute traumatic brain injury and explore its mechanism. Methods Ninety-six male Wistar rats were randomly divided into sham-operated group (S group) and traumatic brain injury group (TBI group).Each group was divided into 6 subgroups (3,6,12,24,48 and 72 h after trauma).Brain injury animal models were established according to Feeney' s method.Measurement of Evans Blue (EB) was performed at different time points after injury to measure the changes of BBB permeability.Brain water content was tested by wet-dry weighting method. Expression of tight junction protein Claudin-5 was detected by Western blotting. Results As compared with that in rats of the S group,the brain water content and content of EB in rats of the TBI group were significantly increased at each time point (P＜0.05); and in TBI subgroups,the brain water content and content of EB were begun to increase 3 h after TBI,reaching its peak level 24 h after TBI.As compared with that in rats of the S group,the expression of Claudin-5 in rats of the TBI group was significantly decreased at 12,24,48 and 72 h after TBI (P＜0.05); and in TBI subgroups,the expression of Claudin-5 was begun to decrease 6 h after TBI,reaching its lowest level 24 h after TBI. Negative correlations were noted between protein expression level of Claudin-5 and both the brain water content and content of EB (r=-0.994,P=0.000; r=-0.846,P=0.036); positive correlation was observed between the brain water content and content of EB (r=0.863,P=0.027). Conclusion The degree of brain injury and changes of permeability of BBB may be time dependent in rats after acute brain traumatic injury; and Claudin-5 may be correlated with the changes of BBB.