The effect of triparanol, an inhibitor of cholesterol synthesis, on the phenobarbital induced proliferation of the smooth endoplasmic reticulum (SER) is reported. Hepatocytes of hamsters treated with phenobarbital alone display the expected proliferation of the SER. Those given triparanol alone do not show this phenomenon but appear similar to controls with the exception of increased numbers of dense bodies characteristic of this treatment. Hepatocytes of hamsters receiving triparanol followed by phenobarbital display a SER proliferation of approximately the same magnitude as that achieved with phenobarbital alone. The effectiveness of the triparanol blockade is documented by the administration of labeled mevalonic acid and the subsequent quantitation of the amount of labeled cholesterol and labeled desmosterol. Desmosterol is the primary end product after triparanol treatment. Greater than 95% of the labeled sterol appears as desmosterol indicating a high degree of blockade. It is concluded that newly-synthesized cholesterol is not a prerequisite for the phenobarbital induced proliferation of the SER. Several possible explanations for these results are discussed.