Objective To evaluate the effect of static magnetic field on the expression of SPmRNA in TG in rats.Methods 44 Wistar rats aged 67 weeks were put into static magnetic field and were sacrificed at 1 h, 2 h, 6 h, 12 h, 24 h, respectively. In situ hybridization method was used to evaluate the changes of SPmRNA expression at different time point.Results Many neurons in TG were marked with SP probes in each group, the expression of SPmRNA increased remarkably in static magnetic field group. In this group, the percentage of SPmRNA positive neurons in TG increased greatly in 1 h, reached its peak in 2 h, from then on, decrease of the percentage started slowly but a moderate percentage was kept until 24 h, which was thought to be enough to maintain orthodontic tooth movement. The tendency of control group was almost the same with that of experimental group. The expression of SPmRNA was higher in experimental group within 2 h but became lower after 2 h as compared with control group, this indicated that magnetic field reduced the SPmRNA expression and exerted restoring effect on trauma. There were significant differences between experimental groups and control group at different time points （P
