Objective:To compare the effects of antenatal ambroxol and glucocorticoid(dexamethasone and betamethasone) on rat fetal lung morphogenesis. Methods:Twelve pregnant rats were randomly assigned four groups(n = 3):ambroxol group,dexamethasone group,betamethasone group and control group. The histologic structures of fetal rat lungs were observed with light microscope,transmission electron microscope and image analysis. Results:Compared with control group,fetal rat lung in the treatment groups had longer respiratory thecal perimeter(P < 0.05),larger alveolar space and thinner alveolar walls(P < 0.01). Fetal lung in the betamethasone injected rats had larger pulmonary alveoli,longer respiratory thecal perimeter and thinner alveolar septum(P < 0.01) than the dexamethasone treated animals. Under the transmission electron microscope,lamellar body and cellular organs in endochylema of typeⅡ alveolar cell of the treatment groups were more common compared with the control group. Obvious changes could be found in the fetal lung of dexamethasone and betamethasone treated animals. Conclusion:All of ambroxol,dexamethasone and betamethasone could promote the development of fetal lung,and the effect of betamethasone could be better than dexamethasone. Ambroxol could be used as an alternative drug to accelerate fetal lung maturity without the side effects of glucocorticoid.