The conversion of 7-dehydrocholesterol to cholesterol is the last reaction in the cholesterol biosynthesis pathway catalyzed by the microsomal enzyme, 7-dehydrocholesterol-delta 7-reductase. We studied whether malignant tumor growth that depends on cholesterol could be slowed by inhibiting late cholesterol biosynthesis. The inhibitor 7-dehydrocholester-delta 7-reductase, BM 15.766 alone, or in combination with 2% cholesterol was fed to 20 male Buffalo rats for 2 weeks immediately after Morris hepatoma 7288CTC was implanted in both flanks. Tumor weights were compared and sterol composition, hepatic hydroxymethyl glutaryl coenzyme A (HMG-CoA) reductase activity, and low- density lipoprotein (LDL) receptor binding in the tumor were correlated with those in the liver. In the plasma of rats treated with BM 15.766, cholesterol levels dropped 75% and the precursor, 7-dehydrocholesterol rose substantially. Tumor weights were 43% less (P < .05) than controls (5.9 ± 1.5 g vs. 10.4 ± 2.2 g) with sterol concentrations reduced 25%, and the precursor, 7-dehydrocholesterol, increased to represent 71% of the tumor sterols. Feeding cholesterol with BM 15.766 normalized plasma but only partially restored tumor cholesterol concentrations, which still remained 49% below the hepatomas in the control group. With BM 15.766, hepatic cholesterol decreased 76% and was associated with a marked rise of 7-dehydrocholesterol that could be almost entirely prevented by feeding cholesterol. After the tumor was implanted, hepatic HMG-CoA reductase activity increased 56% and was 8.6 times higher than in the tumor. Enzyme activities were enhanced about 50% in the liver and the tumor after BM 15.766 was administered but decreased 38% below control when cholesterol was added to the diet. Hepatic receptor-mediated LDL binding rose 67% after tumor implantation, and declined to control levels with cholesterol feeding. These results suggest that de novo cholesterol synthesis in Morris hepatoma 7288CTC is much lower than the liver and tumor growth depends on circulating plasma cholesterol. Inhibiting the last step in cholesterol biosynthesis profoundly reduced tissue and plasma cholesterol concentrations and accumulated precursors substantially to slow hepatoma growth. Feeding cholesterol restored liver but not hepatoma cholesterol levels. Thus, inhibiting late cholesterol synthesis hinders growth of rapidly enlarging malignant tumors.