Mammalian cells obtain cholesterol for membrane synthesis mostly via the receptor-mediated endocytosis of low-density lipoprotein (LDL)¹. Macrophages and vascular endothelium additionally have receptors that recognize certain modified forms of LDL (for example, acetyl-LDL)^2,3. The process by which cholesterol returns from peripheral cells to hepatocytes (reverse cholesterol transport) has not been established; although tissue culture studies have favoured high-density lipoprotein (HDL) as the principal vehicle^4,5, the in vivo evidence for this is meagre. When cholesterol-loaded macrophages are incubated in medium containing plasma, cholesterol moves from the cells to HDL and is then esterified by lecithin/cholesterol acyltransferase⁶. The accumulation of cholesteryl esters in the particles increases their size and decreases their density; enrichment with apoprotein E (apo E) also occurs, producing a decrease in electrophoretic mobility^7,8. We now report that similar changes occur in the circulating HDL of rabbits, when their peripheral tissues are loaded with cholesterol by intravenous (i.v.) injection of acetylated or native human LDL. This result suggests that HDL is involved in reverse cholesterol transport in vivo.