The purpose of this investigation was to determine the effect of a lithogenic diet (one which is known to cause gallstones in hamsters) on the concentration of HMG CoA reductase, the rate limiting enzyme in the synthesis of cholesterol. Syrian hamsters were either fed a normal diet or a lithogenic diet and then at various periods were sacrificed. The hepatic content of HMG CoA reductase was determined. Cholesterol gallstones were produced in the hamsters fed the lithogenic diet and this was accompanied by an increase in the HMG CoA reductase activity of the liver of sacrificed hamsters. The elevation of this enzyme is indicative of increased cholesterol synthesis as a responsible factor in the genesis of cholesterol gallstones in these hamsters.