Dietary fat is, after hydrolysis and reesterification in the intestine, transported by the blood mainly in the form of triglycerides. To enable the transport of the hydrophobic lipid in the aqueous bloodplasrna, a protein envelope is synthesized around the triglycerides in the intestine. The particles formed in this way are called chylomicrons. Endogenous triglycerides synthesized in the liver from free fatty acids and carbohydrates are secreted by the liver in an analogous manner. These liver particles are smaller than the chylomicrons and have a relatively high protein content. They are called very low density lipoproteins (VLDL) . In the breakdown of the triglyceride moiety of both chylomicrons and VLDL an enzyme located at the surface of endothelial cells of bloodvessels, is involved. This enzyme, lipoprotein lipase, is released into the blood after injection of heparin. The measurement of the lipoprotein lipase activity in postheparin serum is an important tool in the study of serum triglyceride metabolism.