The mechanisms for the association of old age with post-prandial hyperlipidemia and atherosclerosis are not well understood. Post-prandial hyperlipidemia has emerged as a significant risk for atherosclerosis. The liver is the central organ for lipoprotein metabolism. The initial step in the hepatic uptake of post-prandial lipoproteins is their transfer from the hepatic sinusoidal capillary lumen across the hepatic sinusoidal endothelium into the space of Disse. Here, they access hepatocytes for receptor-mediated uptake. We proposed that fenestrations (pores) within the hepatic sinusoidal endothelium filter lipoproteins on the basis of size. Recently we discovered age-related changes in the sinusoidal endothelium (pseudocapillarization), including reduction in the porosity of the endothelium. Using the impulse response technique in perfused rat livers, we found that aging is associated with impaired hepatic transendothelial transfer of chylomicrons with diameters smaller than those of fenestrations. In conclusion, age-related pseudocapillarization of the hepatic sinusoidal endothelium provides a novel mechanism for the association of old age with impaired hepatic lipoprotein metabolism and with atherosclerosis.