The activity of beta-hydroxy-beta-methylglutaryl coenzyme A reductase, the rate-controlling enzyme of cholesterol synthesis, was studied in normal livers and in 64 primary hepatocellular carcinomas from rats fed a basal diet or a diet containing either 2% cholestyramine or 5% cholesterol. The average enzyme activity in hepatocellular carcinomas from rats fed the basal diet was more than twice that in normal liver. Dietary cholesterol caused a reduction in activity to one-ninth of the normal hepatic enzyme activity, whereas cholestyramine feeding resulted in a 7-fold increase above the basal level. The data tended to confirm the previously documented observation that "diet-induced feedback inhibition" of cholesterol synthesis is not expressed in hepatomas, since the enzyme activity was reduced only slightly in cancers from rats fed cholesterol. However, the activities from cancers of cholestyramine-fed rats were 2.7 times greater than those from cholesterol-fed rats. Thus, a degree of control was clearly demonstrable, although it represented only 4% of that seen in normal liver. To our knowledge this is the first report of at least partial "feedback control" of beta-hydroxy-beta-methylglutaryl coenzyme A reductase activity in hepatocellular carcinomas grown in vivo.