Objective: Estrogen, the principal female sex steroid hormone, is known to play important physiological roles in bone metabolism. The purpose of this study was to investigate the effects of continuous administration of 17b-estradiol (E₂) on orthodontic tooth movement in relation to its effects on bone turnover. Methods: Thirty 6-week-old female Wistar rats were used. They received orthodontic force for 21 days from an expansive spring placed within the upper first molars. Throughout the experimental period, rats were continuously administered either vehicle (Control group), 0.15mg E₂ /day (Low dose group) or 15mg E₂ /day (High dose group) using osmotic minipumps. Amount of tooth movement was measured on days 0, 1, 3, 7, 10, 14, 17, 21 after the application of force. Serum estradiol and progesterone, serum bone metabolic markers, and bone mineral density (BMD) in the tibial metaphysis were measured at the end of experiment. The data were subjected to the Tukey-Kramer test to identify differences between groups. Results: The continuous administration of E₂ caused a significant and dose-dependent reduction of tooth movement. The amount of tooth movement on day 21 in the High dose group was about 21% smaller than that in the Control group. Serum estradiol levels in the Low and High dose group were 2.5 and 46.5 folds higher than that in the Control group, respectively. Serum progesterone in the High dose group also showed a significant elevation. In contrast to these increases in the hormones, serum levels of TRAP activity, pyridinoline, and osteocalcin significantly decreased in the High dose group, which suggests the inhibition of both bone resorption and bone formation. The administration of E₂ significantly increased the BMD in a dose-dependent manner, which indicates stronger inhibition of bone resorption than bone formation. Conclusion: These results suggest that E₂ inhibits orthodontic tooth movement by inhibiting bone turnover.