Fat emulsions are essentially composed of triglycerides and phospholipids. Their elimination from the plasma--which is generally rapid--is influenced by the amount and the composition of both these components. During their short stay in the vascular compartment, exogenous particles undergo major compositional changes. They acquire various apolipoproteins--namely C-II, C-III, E and A-IV--by transfer from HDL. They also acquire esterified cholesterol from HDL and LDL and transfer exogenous triglycerides and phospholipids to these endogenous lipoproteins. These exchanges are affected by the type of triglyceride fatty acids and the amount of phospholipids present in fat emulsions, as well as by the infusion rate. Some 10% of emulsions--with a high phospholipid: triglyceride ratio--contain a huge phospholipid excess which can be isolated as a separate fraction from the triglyceride-rich particles. These phospholipids markedly interfere with the metabolism of cholesterol and the plasma lipoprotein profile.