Antepartum administration of aminophylline to pregnant rabbits resulted in accelerated formation of phospholipids regarded to be important components of pulmonary surfactant. Increases in the tissue content and synthesis of saturated phosphatidylcholine and phosphatidylinositol were found in lung slice preparations obtained from fetuses previously treated with aminophylline. Synthesis of phospholipids was measured using labeled palmitate, oleate, and glucose as precursors. The results showed increased de novo production of these lipids, and that glucose and glycogen may serve as important precursors. There was a significant reduction in triglyceride content while free fatty acids increased suggesting increased lipolysis in the aminophylline-treated groups, but this did not measurably affect the rate of labeled palmitate incorporation into total phospholipids.