The regulation of the biosynthesis of choline phosphoglycerides during lung development in the rat is investigated with particular emphasis on the formation of dipalmitoyl-phosphatidylcholine. A detailed study of the activity and properties of CTP: choline phosphate cytidyltransferase, CDP-choline: diglyceride transferase and phosphatidic acid phosphohydrolase are currently being investigated. CTP: choline phosphate cytidyltransferase from fetal lung has previously been shown to be activated by inositol phosphoglycerides. Purification of this enzyme is in progress and the purified enzyme will be used to study the mechanisms associated with the activation. Radioactive inositol phosphoglycerides will be prepared and used to investigate phospholipid-protein interaction. The diglyceride substrate specificity for CDP-choline-diglyceride transferase is investigated using both endogenously prepared radioactive substrates and exogenously added radioactive substrates. Emphasis is given to possible substrate specificities associated with dipalmitoyl-phosphatidylcholine formation.