As an extension of metabolic studies of the cholesteryl ester component of rat very low density lipoproteins, we have studied the metabolism of the B apoprotein component labeled by intravenous injection of [3H]lysine. The B apoprotein separated from other apoproteins by delipidation and selective precipitation with tetramethylurea could not be distinguished from B apoprotein prepared by the conventional gel filtration technique. After injection of [3H]lysine, specific activity of B apoprotein was maximal in very low density and low density lipoproteins 1 and 11/2-h later, respectively, in a manner consistent with a precursor-product relationship. When protein-labeled very low density lipoproteins were injected into rats, the relationships of specific activity again indicated that B apoprotein of very low density lipoproteins may be the sole precursor of that of low density lipoproteins. However, less than 10% of the B apoprotein that disappeared from very low density lipoproteins appeared in density lipoproteins. To evaluate the sites of removal of B aproprotein of very low density lipoproteins from plasma, protein-labeled very low density lipoproteins were incubated with unlabeled high density lipoproteins to reduce radioactivity in non-B apoproteins selectively by molecular exchange. Most of the B apoprotein was rapidly removed by the liver. The extensive hepatic uptake of both the cholesteryl ester and B apoprotein components of rat very low density lipoproteins may explain the characteristically low concentrations of plasma low density lipoproteins in the rat.