Lipoprotein concentrations and metabolism were studied in 5- and 9-year-old rhesus monkeys (Macaca mulatta). Both age groups had been divided into control (13.8% of the calories as protein) and low-protein (3.7% protein) subgroups at birth. All were tested before and after their dietary lipid was changed from corn oil to butter plus cholesterol. The concentrations of very-low-density and high density2 lipoproteins (VLDL and HDL2) tended to be higher in monkeys of the low-protein group, and butter plus cholesterol accentuated the difference. All monkeys of the low protein group had elevated levels of at least one of these two classes of lipoproteins. The secretion of nascent VLDL (after intravenous Triton WR-1339) was greater in the low protein than in the control group when both were fed butter plus cholesterol, and rates of VLDL secretion showed a strong positive correlation with the fasting levels of VLDL. Thirty minutes after the intravenous administration of heparin, VLDL triglyceride was almost completely removed, and the apolipoprotein was reduced by 40%; VLDL cholesteryl esters were unchanged. There was a simultaneous decrease in the intermediate density lipoproteins (IDL) and an increase in HDL2, but the low density lipoproteins (LDL) and HDL2 did not change. The rate of VLDL protein removal from plasma was greater in the low protein than in the control group, and the rates for individual monkeys correlated with the levels of VLDL and HDL2 prior to heparin administration. Hepatic postheparin lipase measured in vitro with saturating levels of substrate was significantly higher in the plasma of control than in that of low protein monkeys.