Objective:To observe the effect of dexamethasone(DXM) in the prevention of the development of third molars in SD rats.Methods:12 SD rats of 5 days old(PN 5 d) and 12 SD rats of 21 days old(PN 21 d) were respectively divided into 3 groups(n =4).The rats were injected with 0.1 ml DXM at 1.94 × 10-3 mmol/L,1.94 × 10-4 mmol/L and 1.94 × 10-5 mmol/L respectively into the experimental side,and 0.1 ml normal saline into the control side.PN5d SD rats were respectively killed 21 d and 43 d after injection.PN 21 d SD rats were killed 43 and 52 d after infecion.The PN 21 d SD rats injected with the highest concentration of DXM were the successful models of development prevemion of their third molars in experimental side.The tissue samples were subjected to general observation,radiographic and HE staining examination.Results:After injection,in the highest concentration groups,third molars on the experimental sides of PN 21 d SD rats were not erupted,the control side showed normal eruption.X-ray examination showed crown formation on the experimental sides of the third molars,but without root formation.HE biopsy revealed disorder of root development.Conclustion:DXM with certain concentration injected into local area in late bell stage can prevent tooth development.