[14C]Arachidonic acid conversions were studied in homogenates of lungs from 20, 21, and 22 day fetuses with or without prenatal dexamethasone treatment. The major metabolites were in all cases 12-L-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE), 6-keto-prostaglandin F1α (6-keto-PGF1α), prostaglandin E2 (PGE2), and 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT). Prostaglandin F2α, (PGF2α), prostaglandin D2 (PGD2), and thromboxane B2 (TxB2) were present in small amounts. Dexamethasone treatment significantly stimulated the conversion of [14C]-arachidonic acid in fetal lung homogenates to 6-keto-PGF1α, PGE2, 12-HETE, and HHT, at 20 days gestational age. This effect was dependent on the dose of dexamethasone. These results suggest that dexamethasone accelerates the maturation of the enzymes involved in prostaglandin synthesis. Because dexamethasone is also known to inhibit phospholipase A2, further studies are required to determine the overall in vivo effect of prenatal dexamethasone therapy on fetal lung prostaglandin synthesis.