Objective To elucidate the regulatory bio- mechanism of estrogen and local stress on the bone remodeling activities in vivo. Methods A Wistar rat experimental model was established to apply orthodontic activation during different stages of estrous cycle. In situ hybridization was used to detect the variation of the insulin - like growth factors（IGF）mRNA expression in the periodontal tissues. The data of each group was analyzed with SPSS 11.0 software package for Student-Newman-Keuls（S-N-K）test and one way ANOVA test. Results Both IGF- ⅠmRNA and IGF- Ⅱ mRNA level expressed in the periodontal tissues were increased by orthodontic activation during various stages of the estrous cycle. But a similar pattern emerged that the IGF- Ⅰ mRNA level was the lowest in the estrous stage, whereas its level was the highest in the proestrus stage （P
