Objective To study the changes of the permeability of blood-brain barrier (BBB) in the early stage after cerebral ischemia-reperfusion (I/R) injury in rats. Methods Thirty rats were randomly divided into 6 groups:sham group, ischemia for 1 h group (I1h), ischemia for 2 h group (I2h), ischemia for 2 h followed by reperfusion for 0.5 h (I2h/R0.5h), ischemia for 2 h followed by reperfusion for 1 h (I2h/R1h), and ischemia for 2 h and then reperfusion for 2 h (I2h/R2h). The cerebral ischemia-reperfusion model of middle cerebral artery occlusion (MCAO) was established by middle cerebral artery occlusion. The BBB structure was observed with transmission electron microscopy. After I¹²⁵ labeled nerve growth factor (I¹²⁵-NGF) was injected into the rats by tail vein, its permeation through the BBB was determined by counting γ-rays. Autoradiography was used to observe the distribution of I¹²⁵-NGF in the involved areas in the frozen section of brain coronal tissues. Results Compared with that in rats of the sham group, the content of I¹²⁵-NGF was significantly higher in the rats from the I1h, I2h, I2h/R0.5h, I2h/R1h, and I2h/R2h groups (P<0.05). Compared with sham group, the I2h group displayed that the number of microvilli on the luminal surface of endothelial cells was increased significantly, and the endothelial cells were slightly shrunken, with the pinocytotic vesicles formed in the cytoplasm and slightly swollen mitochondria. And the astrocytes were observed with mild swollen foot processes and wilder extracellular space. In the I2h/R2h group, the endothelial cells were swollen and blurred, with wrinkled nucleus and larger gap, and the astrocytes were with mild swollen foot processes, wilder extracellular space, and accumulated edema fluid. I¹²⁵-NGF was slightly infiltrated into the ventricle of the sham group, moderately into the ventricle and involved cortex of the I2h group, but largely into the ventricle, involved cortex and subcortical tissues in the I2h/R2h group. Conclusion BBB is impaired at the early stage of cerebral ischemia-reperfusion injury in rats. With the elapse of ischemia/reperfusion duration, the integrity of the BBB has deteriorated more severely.