Objective:To investigate the effect of growth hormone on osteopontin expression during orthodontic tooth movement in rats.Methods:Forty male Wistar rats (2 weeks,average weight 200 g) were randomly divided into a control group and an experimental group (n=20 per group).The experimental group received subcutaneous injections of growth hormone at a dose of 0.15 U/(kg·d),and the control group received equivalent volumes of saline.A nickel-titanium spring was fixed between the maxillary incisors and the left upper first molar with a force of 0.49 N to move the molar mesially.All rats were sacrificed on day 3,7,10 or 14 with cardiac perfusion.And the left side of upper jaw was removed.The longitudinal section of the first molar was prepared from sagittal direction to observe osteopontin expression in the pressure area between the mesial buccal root and distal buccal root by immunohistochemical staining and semi-quantitative analysis.Results:The positive expression of osteopontin in the 2 groups showed similar trend,which was increased firstly and then was decreased.The expression of osteopontin on day 7 in the experimental group showed strong positive expression but it was decreased significantly on day 10 compared with the control group (P＜0.05).Conclusion:Systemic application of growth hormone could affect the expression of osteopontin and probably plays an important role in regulating bone resorption in the compression area during orthodontic tooth movement in rats.