Abstract: Objective: To study the effect of chronic stress on the behavior and the estradiol level in different age rats. Methods: Thirty-six two-month-old and fifteen-month-old female Wistar rats were used in the experiments. They were randomly divided into four groups: young normal control(n=8), young stress(n=10), aged normal control(n=8) and aged stress(n=10), respectively. The stress groups were exposed to a chronic unpredictable stress for 21 days, and then they were raised normally for one week. Their behavior was determined with the method of Open field before and right after stress and one week after stress respectively. A blood sample was obtained from the vena caudalis and the serum level of estradiol was detected by radio-immunoassay．Results: Compared with the control groups, the behavioral score and the serum level of estradiol were significantly lower in the stress groups after stress（P＜0.05）, and they decreased more remarkably in the aged group than in the young group（P＜0.01）, which remained at one week after stress（P＜0.05）. Conclusion: Damage induced by chronic stress is different in young and aged rats, and the damages to the behavior and reproductive endocrine system are more serious in the aged rats than in the young, indicating that aging is one important influencial factor in physical stress reactivity.

Key words: Chronic stress, Aging, Estradiol, Behavior