The lanosterol 14 alpha-methyl demethylase inhibitors miconazole and ketoconazole have been used to assess their effects upon cholesterol biosynthesis in cultured Chinese hamster ovary cells. In Chinese hamster ovary cells treated with either agent, an initial accumulation of lanosterol and dihydrolanosterol has been observed. At elevated concentrations, however, ketoconazole, but not miconazole, causes the preferential accumulation of 24,25-epoxylanosterol and squalene 2,3:22,23-dioxide. These metabolites accumulate at the expense of lanosterol, thereby demonstrating a second site of inhibition for ketoconazole in the sterol biosynthetic pathway. Both demethylase inhibitors produced a biphasic modulation of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway. The biphasic modulation is characterized by low levels of the drugs suppressing HMG-CoA reductase activity which is restored to either control or above control values at higher drug concentrations. This modulatory effect of the lanosterol demethylase inhibitors upon HMG-CoA reductase was not observed in the lanosterol 14 alpha-methyl demethylase-deficient mutant AR45. Suppression of HMG-CoA reductase activity is shown to be due to a decrease in the amount of enzyme protein consistent with a steroidal regulatory mechanism. Collectively, the results establish that lanosterol 14 alpha-methyl demethylation, but not 24,25-epoxylanosterol formation, is required to suppress HMG-CoA reductase in the manner described by lanosterol demethylase inhibitors.