The level of dietary protein influenced the rate of cholesterol-4-C14catabolism in bile duct cannulated chicks and rats. Animals which had been kept on a high protein diet for 5 weeks excreted cholesterol-4-C14as bile acids at a faster rate than those on a low protein diet. The presence of dietary cholesterol did not change the rate of cholesterol-4-C14catabolism at either high or low dietary protein levels. A species difference in the rate of cholesterol catabolism was also noted. Bile duct cannulated chicks excreted an average of only 8 per cent, while bile duct cannulated rats excreted as much as 46 per cent of injected cholesterol-4-C14in a 72 hour period. It was shown that cannulation of the bile ducts caused a significant decrease in the serum cholesterol and serum β-lipoprotein levels in chicks. Furthermore, the liver cholesterol level did not seem to influence the rate of cholesterol catabolism. Serum cholesterol, especially in the form of low Sf β-lipoproteins, appeared to be used rapidly for the conversion of cholesterol to bile acids.