Fetal lung fatty-acid synthase and choline-phosphate cytidylyltransferase activities are increased by glucocorticoids. There is evidence that the hormone increases synthesis of fatty-acid synthase but only increases the catalytic activity of the cytidylyltransferase. Free fatty acids and a number of phospholipids have been reported to stimulate cytidylyltransferase activity in several organs, including the lung. We have addressed the question of whether glucocorticoid induction of fatty-acid synthase mediates the stimulatory effect of the hormone on choline-phosphate cytidylyltransferase activity. Explants of 18-day fetal rat lung were cultured for 48 h with dexamethasone and inhibitors of de novo fatty acid biosynthesis (agaric acid and hydroxycitric acid) being included in the medium for the final 20 h. Dexamethasone increased the activities of fatty acid synthase and choline-phosphate cytidylyltransferase by 84% and 60%, respectively. Agaric acid and hydroxycitric acid completely abolished the stimulatory effect of the hormone on cytidylyltransferase but not on fatty-acid synthase. The inhibitors had no effect on cytidylyltransferase activity in control cultures. Fetal lung choline-phosphate cytidylyltransferase can be maximally stimulated by inclusion of phosphatidylglycerol in the assay mixture and under this condition, cytidylyltransferase activity in control and dexamethasone-treated cultures in the presence and absence of the inhibitors were all increased to the same level. Therefore, the inhibitors did not diminish the capacity of cytidylyltransferase to be fully activated. We suggest that the glucocorticoid induction of fatty-acid synthase in fetal lung results in increased synthesis of fatty acids which in turn, either as free acids or after incorporation into phospholipids, activate choline-phosphate cytidylyltransferase.