Rat aortic smooth muscle cells in culture were incubated with rat or human iodinated low and high density lipoprotein at 5-50 mug/ml for 3 h. With the homologous lipoproteins, 25-49% of total cellular protein radioactivity was trypsin releasable and was considered as surface-bound radioactivity, while the balance represented cellular uptake. The ratio of surface-bound to cellular label was higher when the cells were incubated with human lipoproteins and was about 9 : 1 with human high density lipoprotein. Cellular uptake of rat low density lipoprotein was about twice that of rat high density lipoprotein, while degradation of labeled protein, which had presumably followed protein uptake, was similar and ranged from 20 to 25% of protein uptake in 3 h. Experiments designed to test the effect of cell density on lipoprotein uptake have shown that the uptake was related inversely to cell density. Thus, the lower lipoprotein uptake encountered in the rat smooth muscle cells, compared to that described for human fibroblasts (Goldstein, J.L. and Brown, M.S. (1974) J. Biol. Chem. 249, 5153-5162), could be due in part to the much lower cell density used in the latter studies, as well as to cell type and species difference.