Normal monocyte-derived macrophages maintained in medium containing 20% whole serum exhibited a high activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase while expressing appreciable, but not maximal receptor-mediated uptake and degradation of low-density lipoprotein (LDL). Addition of extracellular mevalonate led to a concentration-dependent fall in both processes, although a higher concentration was required to produce the same effect on LDL degradation as on HMG-CoA reductase activity. The true rate of cholesterol synthesis by the cells was already high without mevalonate and was not further increased when the intracellular pool of mevalonate was expanded. After preincubation with lipoprotein-deficient serum receptor-mediated degradation of LDL was increased but HMG-CoA reductase activity was decreased. Both were more sensitive to extracellular mevalonate and a given concentration of mevalonate now produced the same percentage fall in LDL degradation and reductase activity. The amount of cholesterol synthesized by the cells was markedly reduced during incubation with lipoprotein-deficient serum, probably as a result of the decreased reductase activity compounded by a lack of endogenous substrate. If these were circumvented by adding mevalonate, cholesterol synthesis was restored to normal. These results suggest that the fall in HMG-CoA reductase activity during incubation with lipoprotein-deficient serum resulted from repression by some non-sterol product of mevalonate metabolism. They imply that the need for this product by cells growing in whole serum leads to a particularly high HMG-CoA reductase activity and could account for their apparent preference for endogenous synthesis of cholesterol rather than for receptor-mediated uptake of LDL.