Recent data indicate that the kidneys are the most important organ for the catabolism of HDL-apolipoproteins in the rat. This catabolic process is probably initiated by a specific HDL receptor in kidney membranes. However, it is unlikely that receptor-mediated endocytosis of intact HDL is involved in HDL-apolipoprotein degradation by kidneys, since HDL-cholesteryl esters are not taken up by the kidneys in vivo. It is proposed that the kidneys catalyze the first rate-limiting step in the degradation of HDL by "stripping" and degradation of a part of the HDL-apolipoproteins. In vivo metabolism of HDL-cholesteryl esters and LDL occurs predominantly in the liver and not to any significant extent in the kidneys.