OBJECTIVE To investigate the effect of estrogen replacement therapy on novel-object recognition and extinction of conditioned fear memory in ovariectomized (OVX) rats.METHODS Sixty sexually mature (> 90 d) female SD rats weighing 250-350 g were used as subjects.The rats were ran domly divided into 7 groups as follows : OVX with low dose of es trogen replacement (OVX + LD, 0.1 mg· kg-1, n =9), medium dose (OVX + MD, 0.3 mg· kg-1, n =9) and high dose (OVX + HD, 0.9 mg· kg-1, n =9), estrogen replacement with medi um dose at 2 weeks after OVX (MD + 2W, n =9) and 4 weeks after OVX (MD +4W, n=8), OVX group (n =8) and Sham group (SH, n =8).One week after arrival, all rats underwent surgery, and after at least 1 week of recovery from surgery, rats were allowed to experiment.According to the different dose and time schedules, each group was administrated estrogen or vehicle via subcutaneous injection on the dorsal side of the rat.Estrogen maintenance one month after ovariectomized surgery, then, the Novel-object recognition and extinction of conditioned fear memo ry were tested.RESULTS The rats after OVX showed access dependent impairments on novel-object recognition and the estro gen replacement therapy can reversal this phenomenon, but not the HD group.Simultaneously, the effect is associated with the time schedules of estrogen, the earlier the better.On extinction of conditioned fear memory, the OVX rats produced significandy less freezing by fear context than that of SH rats, after chronic administration with estrogen one month exhibited the enhanced acquisition of context fear, except HD group.Estrogen replace ment therapy also increased the speed of extinction of conditioned fear memory.CONCLUSION These results indicate that es trogen may have an effect on the central nervous system, and it plays an important role for improving emotional learning and memory during deprive of sex hormone.