In sham-operated rats, intravenous administration of 14C-very low density lipoprotein triglycerides (with labelled esterified fatty acids) caused an initial decrease and subsequent increase in plasma 14C-lipids of both very low density lipoproteins (VLDL) (density < 1.006) and lipoproteins of density > 1.019. There was a similar change in 14C-lipids in adipose tissue and heart whereas in kidney, spleen and liver, 14C-lipids increased initially and then decreased. Insulin treatment in sham-operated animals decreased circulating 14C-lipids in VLDL and in lipoproteins of density > 1.019, while intermediate density (1.006-1.019) lipoproteins increased. Insulin also enhanced the radioactivity retained in spleen. In functionally hepatectomized rats, 14C-lipids progressively increased in heart. Insulin treatment in these rats enhanced the disappearance from circulation of 14C-VLDL and of lipoproteins of density > 1.019, as well as the appearance of 14C-intermediate density lipoproteins. Toe appearance of 14C-lipids in white adipose tissue also was augmented, while it decreased in heart and lung. Thus, in sham-operated animals, insulin apparently stimulates the uptake of products of VLDL metabolism by cells in the reticuloendothelial system, while in functionally hepatectomized rats there is increased heart utilization of VLDL triglycerides, and insulin enhances the net extrahepatic catabolism of these lipoproteins.