Abstract: Objective Establish depression model of rat through chronically unpredicted mild stress（CUMS）in order to investigate its connection with the changes of hippocampal pathomorphology and Brain-derived neurotrophic factor（BDNF）. Methods Rats were randomly divided into two groups,control and model.CUMS was used to establish depression model of rats. The rat behavior, SOD activity and MDA level of cortex, hippocampal pathomorphology, and BDNF expression were tested by the methods of open-field, xanthine oxidase, TBA,HE staining,and immunohistochemisty, respectively. Results It was shown that there existed significantly decreasing open-field scores，SOD activity，and BDNF expression(A value 803±422 vs 237±96，p<0.01) with obviously increasing MDA level and karyopyknosis of hippocampal neurocytes in rats treated by CUMS，compared with control rats. Conclusions Depression behavior of rats can be induced by CUMS，the mechanism of which may be associated with damage of hippocampal neurocytes and decrease of BDNF expression from imbalance of oxidation-antioxidation system and increase of free radicals in rat brains.

Key words: BDNF