Aim study the change of expression of matrix metalloproteinase-9(MMP-9)and the permeability of blood brain barrier(BBB)in the rats model with cerebral ischemic and reperfusion.Methods A right middle cerebral artery ischemia-reperfusion models were established in this experiment,the change of the permeability of BBB had been evaluated by mensurating the content of Evan's blue(EB),The expression of MMP-9 in ischemia reperfusion rat brains had been investigated by immunohistochemistry technology.Result After cerebral ischemia-reperfusion,the mount of EB exudation of the brain tissues begin increase in 3 h(0.18±0.08 vs 0.86±0.12 μg/g),and will get its peak at the time of 1 d(4.64±1.27 μg/g),and will decrease gradually 2 d later(4.48±0.65 μg/g).The expression of MMP-9 increases notablely at 12 h(3.64±0.56 vs 6.76±2.26),1 d(11.24±2.64),2 d(21.52±2.03),4 d(16.66±1.38),and 7 d(14.64±3.77)later after ischemia-reperfusion,and will go to its peak at the second day.Conclusion The expression of MMP-9 positive cells presents time-related with the increase of BBB permeability after cerebral ischemia-reperfusion,which point out the MMP-9 participated the process of the damage of BBB.