Objectives: To explore the effect of the orthodontic treatment and the timing of applying force on female patients, so as to offer an experimental princinples for women's orthodontic treatment. Methods: 120 female 3-month-old Wistar rats were used. They were randomly divided into control group, sham-treated-group and loading-force group. Each group was divided again into 4 sub-groups according to the different stage of the estrous cycle. The loading-force group received repeated intermittent orthodontic force for four times separately during the same stage of the estrous cycle. Serum estradiol was measured by Radioimmunoassay. The vaginal smears were obtained daily to record the duration of the estrous cycle and each stage of a cycle. The data were analyzed by One-way ANOVA ,SNK and chronobiological statistical methods. Results: There were significant variations in the estradiol level among the groups which received the mechanical force during the same stage of the estrous cycle (p<0.01). There were significant variations in the estradiol level within each group (p<0.05). The highest was in proestrus and lowest in estrus. For the sham-treated group and the groups receiving forces in proestrus and estrus, the duration and rhythm of estrous cycle had not been changed. For the groups receiving forces in metestrus and diestrus, their diestruses were prolonged during the 1st cycle. After about 5 days, the duration and rhythm of estrous cycle returned to normal again. Conclusion: As far as the effects of tooth movement on the estrous cycle and estrogen were concerned, estrus (the stage of lowest level of estradiol) was the best interfering time for the orthodontic treatment. Supported by a grant (NO.30371560) from Natural Sciences Foundation of China.

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