The studies described here suggest the potential physiological role of polypeptide and corticosteroid hormones in the regulation of cholesterol synthesis. Evidence was shown for substantial differences between various cell types in their responses to these agents and for certain degree of independence of the effects on biosynthesis of cholesterol from those on protein and DNA synthesis. Cholesterol synthesis and HMGCoA reductase are stimulated in a number of diploid cell lines following an incubation with insulin or with glucocorticoids for 4 hr or longer. Stimulation of sterol synthesis by insulin and by dexamethasone requires protein synthesis, but the two hormones do not compete for the same site. Addition of glucagon or of dibutyryl cyclic AMP, or elevation of intracellular cyclic AMP by PGE1 does not inhibit cholesterol synthesis in skin fibroblasts. A possibility of a relationship between the mechanisms of the hormonal effects and of feedback control of cholesterol synthesis is suggested.