Biliary secretion of the cholesteryl ester moiety of (modified) low-density lipoprotein (LDL) was examined under various experimental conditions in the rat. Human LDL or acetylated LDL (acetyl-LDL), radiolabelled with [3H]cholesteryl oleate, was administered intravenously to unanesthetized rats equipped with permanent catheters in the bile duct, duodenum and heart. LDL was cleared relatively slowly from plasma, mainly by Kupffer cells. At 3 h after injection, only 0.9% of the radioactivity was found in bile; after 12 h this value was 4.5%. Uptake of LDL by hepatocytes was stimulated by treatment of the rats with 17 alpha-ethinyloestradiol (EE; 5 mg/kg for 3 successive days); this resulted in a more rapid secretion of radioactivity into bile, 3.9% and 12.4% after 3 h and 12 h respectively. The extremely rapid uptake of acetyl-LDL via the scavenger pathway, mainly by endothelial cells, resulted in the secretion of only 2.1% of its 3H label into bile within 3 h, and 9.5% within 12 h. Radioactivity in bile was predominantly in the form of bile acids; only a small part was secreted as free cholesterol. However, the specific radioactivity of biliary cholesterol was higher than that of bile acids in all three experimental conditions. EE-treated animals did not form cholic acid from [3H]cholesteryl oleate, which was a major product of the cholesteryl oleate from LDL and acetyl-LDL in untreated rats, but formed predominantly very polar bile acids, i.e. muricholic acids. It is concluded that uptake of human LDL or acetyl-LDL by the liver of untreated rats is not efficiently coupled to biliary secretion of cholesterol (bile acids). This might be due to the anatomical localization of their principal uptake sites, the Kupffer cells and the endothelial cells respectively. Induction of LDL uptake by hepatocytes by EE treatment warrants a more efficient disposition of cholesterol from the body via bile.