The glycosylation of apo very low density lipoproteins (apo-VLDL) in vivo was studied by following the incorporation of [14C]glucosamine into several groups of apoproteins of VLDL isolated from hepatic Golgi fractions and from serum of sucrose-fed, colchicine-treated rats. Simultaneous incorporation of [3H]leucine was used to quantitate the apoproteins following separation by polyacrylamide gel electrophoresis. Experimental conditions were selected so that the 14C:3H ratio in the apoproteins permitted estimations of the extent of glycosylation by glucosamine and its metabolites. A rapidly decreasing 14C:3H ratio was noted in serum apo-VLDL for the first 30 min after administration of the isotopically labelled precursors, followed by stabilization of the ratio. These data are consistent with the glycosylation of a preformed pool of apo-VLDL, probably apo-B. Glucosamine was progressively incorporated into apo-VLDL during transition from the forming face of the Golgi apparatus to the secretory vesicles, as indicated by an increasing 14C:3H ratio. On the other hand, the ratio of the rapidly migrating apoproteins of VLDL, corresponding to the apo-C-II and apo-C-III, showed the opposite trend, as did total apo high density lipoprotein (apo-HDL) and the rapidly migrating bands of apo-HDL. Division of the rapidly migrating apoproteins of VLDL into upper bands (probably apo-C-II and apo-C-III-0) and lower bands (probably apo-C-III-3) resulted in a 14C:3H ratio near zero in the upper band apoproteins, consistent with the absence of carbohydrates. The lower band showed a rising 14C:3H ratio during transition through the Golgi apparatus, suggesting increased glycosylation, The decreasing 14C:3H ratio in the rapidly migrating proteins is therefore due to the acquisition of apo-C-II and apo-C-III-0 by VLDL during passage from the forming face to the secretory vesicles of the Golgi apparatus.