Primary monolayers of calf aortic endothelial cells were presented with isolated human very low density lipoproteins that had been labeled with radioactive triglyceride. The cells were observed to take up triglyceride over a 24 hr period; incorporation increased with exogenous lipoprotein concentrations, and up to 60% of the triglyceride taken up was converted to other cell lipids within 24 hr. When [2-3H]glyceryl tri[1-14C]oleate-labeled very low density lipoprotein was used, the 3H/14C ratio in the cell triglyceride was always similar to that of the exogenous lipoprotein triglyceride. Moreover, no significant hydrolysis of the exogenous very low density lipoprotein triglyceride was observed during the time of exposure to the cells. Similar experiments using doubly-labeled triglyceride exposed to endothelial cells in triglyceride-phospholipid liposome preparations also resulted in incorporation of the exogenous triglyceride without evidence of extracellular hydrolysis. The results indicate that primary monolayers of endothelial cells in culture are able to incorporate and metabolize very low density lipoprotein triglyceride. However, triglyceride does not appear to be significantly hydrolyzed during uptake, suggesting an absence of lipoprotein lipase activity in these cells.