The effect of i.v. injection of mevalonate on the activity of microsomal 3-hydroxy-3-methylglutaryl Coenzyme A reductase was studied in livers from non-tumor-bearing rats and in host liver and hepatomas from rats bearing transplantable Morris hepatoma 7800. We confirmed that a single bolus injection of 100 mg of mevalonate in non-tumor-bearing male rats caused a 90% inhibition of hepatic 3-hydroxy-3-methylglutaryl Coenzyme A reductase activity within 2 hr. In two experiments mevalonate injection caused a 50 to 60% reduction in enzyme activity of hepatomas but no significant decline in the enzyme activity in host livers. Thirty in after injection of [14C]mevalonate in a similarly sized bolus, the ratio of specific activities of cholesterol in liver:hepatoma:kidney:blood was 13:5.6:0.5:1. Thus, both the liver and hepatoma efficiently utilized mevalonate for the synthesis of cholesterol. The precise cause of the inhibition of enzyme activity in the liver of non-tumor-bearing rats and in the transplantable hepatomas is not clear from this study. However, on the basis of other published reports, we suggest that it resulted from the accumulation of endogenous cholesterol in microsomal membrane. The activity of cholesterol 7 alpha-hydroxylase, the rate-controlling enzyme for bile acid synthesis, was also studied in the hepatoma, but, in general, it did not differ from that in the host liver or control liver.