Using I131 VLDL selectively labelled in the B-apoprotein and I125 LDL injected simultaneously into the patient we have derived some quantitative measures of VLDL and LDL metabolism in man. The effects of insulin resistance, associated with idiopathic hypertriglyceridaemia, adult onset diabetes and diabetic lipodystrophy on the metabolic behaviour of these molecules were also assessed. In the normal subjects 72-83% of the total daily plasma VLDL B-apoprotein flux was metabolised via a pathway which involved its ultimate conversion to plasma LDL, while 21-28% was degraded without such conversion. The amount of B-apoprotein metabolised by either of these routes was proportionate to the flux rate and the two pathways accounted for the total VLDL B-apoprotein removed from the plasma. In patients with idiopathic hypertriglyceridaemia and in the adult onset diabetics the total plasma VLDL B-apoprotein flux was higher than normal, indicating increased production of this apoprotein. On the other hand, the flux rate of plasma VLDL B-apoprotein in the patients with diabetic lipodystrophy was normal, suggesting that the increase in the circulating mass of these molecules was due to impaired clearance. In all the patients, however, the fractions of the total flux either converted to LDL or degraded were lower than normal, suggesting that insulin resistance limited the removal of this apoprotein by these pathways. The results also indicate that a fraction of the total VLDL removed from the plasma has been retained in an extravascular compartment, possibly representing VLDL molecules trapped in the vascular structures. In the control and the insulin resistant subjects the quantity of LDL apoprotein catabolised per day agreed closely with the amount derived from VLDL B-apoprotein conversion, suggesting that VLDL-B-apoprotein serves as the main source of LDL apoprotein. In patients with idiopathic hypertriglyceridaemia and in adult onset diabetics the absolute turnover rate of plasma LDL apoprotein was higher than normal, while in the lipodystrophic patients it was reduced. It is suggested that the increase in LDL turnover seen in the former groups could be an additive factor in the deposition of lipid rich material in arterial walls.