To determine the effects of maternal administration of dexamethasone (DEX) and thyrotropin-releasing hormone (TRH) on fetal lung maturation, 16 pregnant rats were divided into the following four groups: 20 micrograms/kg TRH twice a day was given intraperitoneally to the TRH group rats, 0.5 mg/kg/day DEX to the DEX group and both DEX and TRH to the DEX + TRH group for 3 consecutive days from gestational day 17. The control rats were given an equivalent volume of saline. The pregnant rats were sacrificed on gestational day 20 and the fetal lungs were removed. The relative amounts of surfactant protein A (SP-A), B (SP-B) and C (SP-C) mRNAs were analyzed by Northern blotting and the total lung disaturated phosphatidylcholine (DSPC) contents were determined using an enzymatic method. The SP-B and -C mRNA and DSPC contents in the DEX and DEX + TRH groups were significantly higher than those in the control group, whereas the SP-A mRNA levels did not differ significantly among the four groups. The SP-B and -C mRNA and DSPC contents in the DEX and DEX + TRH groups did not differ significantly. These findings suggest that TRH has no effects on the regulation of surfactant protein mRNAs or DSPC contents in the fetal rat lung and has no additive effects when combined with DEX.