1. Pulmonary maturation in the rat is accompanied by a 30% postnatal increase in the pool size of choline, a 4-fold overall prenatal and postnatal decrease in the level of cholinephosphate, a 3-fold decrease in CDPcholine levels and a 2-fold increase in the content of phosphatidylcholine. 2. The level of 1,2-diacyl-sn-glycerol in rat lung increases 5-fold during the fetal and neonatal periods. Only minor alterations were noted in the fatty acid composition. 3. These results are consistent with an increase in the relative rates of the cholinephosphate cytidylyl-transferase and cholinephosphotransferase steps of phosphatidylcholine production during pulmonary maturation. The relative rate of the step catalyzed by phosphatidate phosphohydrolase may also be increased.