Electrophoresis of rat apolipoprotein B (apoB) on 5% polyacrylamide gels in the presence of NaDodSO4 separates three major components: PI, which comigrates with human low density lipoprotein (LDL) apoB; PII, a slightly faster-moving satellite band; and PIII, which migrates somewhat more slowly than myosin heavy chain. The proportion of PIII decreases with increasing density of the parent rat lipoprotein, from 90% an 70%, respectively, in chylomicrons and very low density lipoproteins (VLDL), to 7% in the major LDL2 (density 1.038-1.063 g/ml) fraction. A major component that comigrates with rat PIII is a marker for human chylomicron apoB, being absent from human VLDL, intermediate density lipoprotein (IDL), and LDL. Preliminary immunological and peptide mapping data show that rat apoB PI and PIII are closely related structurally, with the latter possibly being a large fragment of the former. Both peptides are synthesized in rat liver and found in Golgi secretory vesicles. Kinetic tracer experiments show that rat PI and PIII are present on separate VLDL particles, both of which are extensively removed from the circulation at the remnant stage, and that the declining PIII-to-PI/II ratios in IDL and LDL may be attributed to the more rapid turnover of PIII-containing lipoproteins at all levels, particularly within the LDL density range.