Isolated livers from rhesus monkeys (Macaca mulatta) were perfused in order to asses the nature of newly synthesized hepatic lipoprotein. Perfusate containing [3H]leucine was recirculated for 1.5 hr, followed by an additional 2.5-hr perfusion with fresh perfusate. Equilibrium density gradient ultracentrifugation clearly separated VLDL from LDL. The apoprotein composition of VLDL secreted by the liver was similar to that of serum VLDL. The perfusate LDL contained some poorly radiolabeled, apoB-rich material, which appeared to be contaminating serum LDL. There was also some material of an LDL-like density, which was rich in radiolabeled apoE. Rate zonal density gradient ultracentrifugation fractionated HDL. All perfusate HDL fractions had a decreased cholesteryl ester/unesterified cholesterol ratio, compared to serum HDL. Serum HDL distributed in one symmetric peak near the middle of the gradient, with coincident peaks of apoA-I and apoA-II. The least dense fractions of the perfusate gradient were rich in radiolabeled apoE. The middle of the perfusate gradient contained particles rich in radiolabeled apoA-I and apoA-II. The peak of apoA-I was offset from the apoA-II peak towards the denser end of the gradient. The dense end of the HDL gradient contained lipoprotein-free apoA-I, apoE, and small amounts of apoA-II, probably resulting from the relative instability of nascent lipoprotein compared to serum lipoprotein. Perfusate HDL apoA-I isoforms were more basic than serum apoA-I isoforms. Preliminary experiments, using noncentrifugal methods, suggest that some hepatic apoA-I is secreted in a lipoprotein-free form. In conclusion, the isolated rhesus monkey liver produces VLDL similar to serum VLDL, but produces LDL and HDL which differ in several important aspects from serum LDL and HDL.