A fenestrated endothelial lining of sinusoids in rat liver has been shown to separate chylomicrons of different sizes following their injection into the portal vein. This sieving may have physiological importance, since during low dietary fat intake some intestinal lipoproteins are probably small enough to contact liver cells, but during high dietary fat loads most chylomicrons are too large to pass through the filter and must first be degraded to smaller remnants. The liver plays a central role in cholesterol metabolism since it catabolises dietary cholesterol which inhibits synthesis of cholesterol to be circulated as liver-derived very low density lipoproteins (VLDL) and low density lipoproteins. The sieving of chylomicrons, remnants and other lipoproteins by the sinusoidal endothelium of the liver may thus play an important role in lipid transport, affecting the balance of various lipoprotein moieties which in turn may affect the relationship of dietary lipids to various hyperlipidaemias and atherosclerosis.