C ompactin (ML-236B) and the related compound, mevinolin, are competitive inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase), the rate-controlling enzyme in cholesterol synthesis. Previous studies have shown that administration of compactin to cultured cells elicits a compensatory increase in the amount of HMG CoA reductase in the cells. A similar increase in HMG CoA reductase has been reported in livers of rats and mice that have been treated with compactin. In this study, we explore the mechanism for the mevinolin-mediated increase in hepatic HMG CoA reductase in mice that have been fed a control diet and a 2% cholesterol diet. Administration of mevinolin to mice on a control diet produced a 6- to 10-fold increase in the amount of HMG CoA reductase in liver microsomes. When mice were fed the cholesterol-enriched diet, cholesterol accumulated in the liver and HMG CoA reductase declined by 90%. The administration of mevinolin to cholesterol-fed mice produced a three to eightfold increase in HMG CoA reductase. Despite the abundant amount of cholesterol that was already present in the livers of the mevinolin-treated, cholesterol-fed animals, their elevated HMG CoA reductase could be rapidly suppressed by the subcutaneous injection of small amounts of mevalonate, the product of HMG CoA reductase. These data are compatible with the existence in mouse liver of a multivalent feedback regulatory mechanism for HMG CoA reductase in which suppression of the enzyme requires both a sterol and a nonsterol substance derived from mevalonate. By blocking mevalonate synthesis, mevinolin activates this regulatory mechanism, and this in turn causes an increase in hepatic HMG CoA reductase. The ability to suppress the elevated HMG CoA reductase with mevalonate may prove useful in potentiating the effectiveness of mevinolin as a hypocholesterolemic agent.