Abstract: ABSTRACT : Objective To study change of BDNF expression in hippocampus and prefrontal lobe of rats by acute and chronic forced-swimming stress , and study the effect of different intensity stress on the behaviors of rats. Method After different intensity stress by forced swimming in the water, the change of BDNF mRNA expression in hippocampus was observed by RT-PCR, and Western blotting was used to explore the expression of BDNF protein in rat hippocampus. Result Acute stress resulted in elevated expression of BNDF mRNA and protein, and the expression of BDNF became lower and lower after 3 hours . Meanwhile the chronic stress led to the lower level expression of the BDNF compared with acute group in many time points (p<0.05).Conclusion Acute stress resulted in the high level expression of BDNF in hippocampus. Chronic repeated stress led to the habituation expression of BNDF, which was lower than that of acute stress group.