The present work tested the inhibitory effects of bile acids on the cholesterol biosynthesis and the activity of HMG-CoA reductase in cultured rat hepatocytes. The uptake of bile acids in hepatocytes were increased in according to the different bile acid concentrations and culture times. The rate of cholesterol synthesis in cells were inversely decreased to the bile acid concentrations and culture times. As expected, insulin injection (4 units/100g body weight) showed an enhancing effect of the cholesterol synthesis and the HMG-CoA reductase activity. The addition of bile acids in medium of insulin-treated hepatocytes also showed the suppressing effect. This effect was directly confirmed in isolated hepatic icrosomes by the test of HMG-CoA reductase activity. In the test of $Na^+$,$K^+$-ATPase activity in the isolated hepatocyte membrane, only the cholic acid did not stimulate the enzyme system. The reason of such difference is not obvious, but this result indicates that the cholic acid could be absorbed by simple diffusion.