High affinity cell surface receptors for low density lipoproteins (LDL) are inducible in cultured human lung fibroblasts by the removal of lipoproteins from the cell culture medium. The binding, uptake, and degradation of 125I-LDL by fibroblasts decrease with increasing number of population doublings. The affinity of LDL receptor binding, however, remained unchanged at different population doublings levels. Hence, the difference in LDL binding activity in the aging fibroblasts can be attributed to a reduction in the number of receptor sites on the cell membrane. Cellular uptake of [4-14C]cholesterol and 2-deoxy-D-[1-14C]glucose mediated through mechanisms independent of the LDL receptor pathway revealed no significant difference in early and late passage fibroblasts. This suggests that the alteration in the LDL receptor binding in serially passaged fibroblasts is an "age-related" phenomenon. The late population doubling fibroblasts require more LDL in the culture medium for feedback inhibition of LDL receptor synthesis. Thus, aging fibroblasts are both progressively less inducible and less suppressible in the regulation of their cell membrane LDL receptors. Similar results were also obtained with respect to the regulation of DL-3-hydroxy-3-methyl-glutaryl coenzyme A reductase in the aging fibroblasts in culture; the enzyme has become less inducible and less supressible as the fibroblasts approach the limit of their in vitro lifespan. These age-related alterations in the cellular metabolism of LDL and cholesterol might contribute to our understanding of the increased risk of athlerosclerosis in our aging population.