The present study was undertaken to study the effect of corticosteroids on lecithin production in fetal lung. Rabbit fetuses were injected intraperitoneally with hydrocortisone succinate at 24 days of gestation and sacrificed at 27 days. Lung DNA concentration was not changed, but evidence is presented to suggest that total lung DNA content was decreased by saline injection and by steroid injection. Minimum surface tension of pooled lungs was increased by control injections but was reduced to ‘normal’ levels by the steroids. Steroids caused no significant change in lung total phospholipid or lecithin contents. The specific activity of labelled choline, methionine and palmitate indicated increased incorporation of choline and palmitate into lung lecithin in steroid-injected fetuses. It is suggested that steroids accelerate fetal lung development by influencing the composition of surfactant at the alveolar surface.