The effects of dietary fish, soybean protein and casein on cholesterol turnover were compared in rats. After the injection of [¹⁴C]cholesterol into the rats, the specific activities of radioactive cholesterol in feces were followed for 4 weeks. The cholesterol half-lives calculated from the decay curves of the specific activities were 14.7 and 14.6 days in rats fed fish protein and soybean protein, respectively. These were shorter than the half-life (17.4 days) in casein-fed controls. The fish and the soybean protein feedings significantly increased the fecal excretions of cholesterol and coprostanol, respectively, and lowered the plasma cholesterol level, as compared with casein feeding. In addition, both fish and soybean protein feedings also increased the excretion of bile acids. The stimulation of cholesterol metabolism and the increased excretions of cholesterol and its metabolites by feeding fish or soybean protein appear to play important roles in the hypocholesterolemic effects.