Fat derived from milk triglycerides represents the major energy source during the suckling period in many species. Previous studies have shown several differences between fat digestion, absorption, and transport in neonates compared to adults. However, to date no comprehensive investigation of these processes have been carried out. Using the suckling rat, the objectives of this proposal are: 1) to asses the role of biliary phosphatidylcholine in fat absorption and chylomicron formation; 2) to investigate the biosynthesis of biliary phosphatidylcholine by neonatal liver; 3) to investigate lipoprotein metabolism by the neonatal liver, and in particular to characterize the types of lipoproteins synthesized by the liver and the types of lipoproteins taken up by the liver; 4) to investigate the metabolism of the lipoprotein apoproteins by the suckling rat, and in particular to assess the role of the liver and intestinal tract in apoprotein biosynthesis; 5) to investigate lipoprotein metabolism by heart and skeletal muscle; 6) to investigate the origin and metabolism of plasma low density lipoproteins (LDL), whose concentration is elevated in the suckling rat; 7) to investigate changes in lipoprotein metabolism which accompany weaning and maturation. It is anticipated that these studies will provide badly needed information about fat metabolism during the suckling period, which is a critical time of growth and development.