Objective:To compare the effects of different dosage of dexamethasone(DEX) on rat fetal lung morphogenesis. Methods:Fifteen pregnant rats were divided into five groups with 3 rats in each group randomly:small dosage DEX single and multiple course groups,large dosage DEX single and multiple course groups and control group. On the 19th day of gestation,cesareans were taken out,and the histologic structures of 6 fetal rat lungs in each pregnant rat were observed with light microscope and electronic microscope. Results:Under the light microscope,fetal rat lung in the four treatment groups had fewer alveolar numbers,larger alveolar space,and thinner alveolar septum compared with control group(P < 0.01). With the increased dosages and times of the DEX,the fetal rat lungs have fewer alveolar numbers,larger alveolar space,and thinner alveolar septum(P < 0.01). Under the electronic microscope,lamellar body and cellular organs such as bioblast in endochylema of the four treatment groups were more obvious compared with the control group. But basal membrane of lung epithelial cells turned out to be discontinuous and different thickness.Conclusion:Using DEX antepartum could promote fetal lung morphogenesis obviously. But with the dose and course increased,the development alveoli was inhibited and the basal membrane was damaged.