Objective：To determine the effects of different degrees of chorioamnionitis（CA） and prenatal glucocorticoid on fetal lung maturation and development in preterm rats. Methods：Totally 36 pregnant SD rats were randomly assigned to 6 groups. A group：normal saline（NS）+dexamethasone（DEX）；B group：lipopolysaccharide（LPS） 0.5 μg DEX；C group：LPS 4.0 μg DEX；D group：NS NS；E group：LPS 0.5 μg NS；F group：LPS 4.0 μg NS. All groups underwent intraamniotic injection with NS，LPS（0.5 μg and 4.0 μg respectively） on gestational day 19；24 h later，rats were intramuscularly injected with DEX 0.4 mg and NS respectively. After another 24 h，pathological inflammation of placenta，chorioamnion and fetal lung were observed. Surfactant protein A，B，C（SP-A，SP-B，SP-C） and interleukin-1β（IL-1β），tumor necrosis factor-α（TNF-α） mRNAs in lung were determined by quantitative real-time PCR（qRT-PCR）. Lung morphologic observation was performed on postnatal day 15（P15）. Results：①fetal rat mortality in C，F groups were higher than that in other groups；rat survival rate in C，F groups were lower than that in other groups（P all <0.05）. ②Pathologi－cal grading of pulmonary inflammation and CA：C and F groups were the highest，followed by B and E groups and A and D groups were the lowest（P all <0.05）. ③Relative expression of SP-A，SP-C mRNAs：D and F groups were lower than other groups，B group was the highest（P all <0.05）；relative expression of SP-B mRNA：D，E，F groups were lower than other groups，B and C groups were the highest（P all <0.05）. ④Relative expression of IL-1β mRNA：F group was the highest（P<0.05）. Relative expression of TNF-α mRNA：C and F groups were the highest while A and D groups were the lowest（P<0.05）. ⑤On P15，C group alveolarization was significantly inhibited. Conclusions：Different doses of intraam－niotic LPS could induce CA and fetal lung inflammation with different degrees. Histological inflammation aggravates with LPS dose increasing. Mild to moderate CA and antenatal DEX will not adversely affect the survival rate nor inhibit fetus and newborn growth. Inflammation was not suppressed but synergistically induced more significant lung maturation. Severe CA and prenatal DEX might cause adverse effects，particularly，increasing the risk of bronchopulmonary dysplasia.