Exposure to 5 mg HF/m3 causes a significant increase in the plasma cholesterol levels in the guinea pig. Modifications of the cholesterol metabolism are due to the specific action of fluoride. Effects of HF on cholesterolemia are reversible however, and during a second exposure to HF, plasma cholesterol increases as in the first exposure. Cholesterol biosynthesis was studied, Acetate incorporation in intestinal tissue and lung was higher in intoxicated animals than in controls but mevalonate incorporation was comparable in the two groups. The enzyme catalyzing mevalonate synthesis, 8-methyl-8-hydroxyglutaryl CoA reductase, could be activated by HF.