The catabolism of human and rat 125I-labelled very low density lipoproteins (VLDL) was compared by perfusing the lipoproteins through beating rat hearts. Triacylglycerol was removed from the VLDL to a greater extent than the protein moiety, leaving remnants containing relatively more apo-B and less apo-C. The change in apo-C content of the remnants correlated with the loss of triacylglycerol. The extent of removal of triacylglycerol from the rat and human VLDL was similar and in most cases appeared to saturate the heart lipoprotein lipase. The remnants were slightly smaller in size than the VLDL, and included particles which appeared to be partially emptied. In addition to remnants of d less than 1.019 g/ml, iodinated lipoproteins derived from rat and human VLDL were recovered at d 1.019-1.063 and 1.063-1.21 g/ml. The former contained largely cholesterol and cholesteryl esters, while phospholipids were the dominant lipid in the latter. An average of 40% of the 125I-labelled apoprotein lost from the VLDL was associated with the perfused hearts. Very little d 1.019-1.063 g/ml lipoprotein was produced from low (physiological) concentrations of rat VLDL, most of the lipoprotein being removed by the heart. However, lipoproteins of density 1.019-1.063 g/ml were formed from human VLDL at all concentrations in the perfusate, as well as from higher concentrations of the rat VLDL. Agarose gel filtration of lipoproteins following heart perfusion with human VLDL revealed large aggregates containing particles which resemble low density lipoproteins (LDL) in electron microscopic appearance and apoprotein composition, since they contain largely apo-B. These data suggest that at normal concentrations rat VLDL are almost completely catabolised and taken up by the heart without the formation of LDL, while LDL is produced from human VLDL at all concentrations.