The effect and mechanism of two types of rice protein, one from regular japonica rice Koshihikari and another from rice cultivar Shunyo, with low glutelin and high prolamin content, on cholesterol and triglyceride metabolism were compared by feeding casein and soy protein to male Wistar strain rats 7 and 20 weeks old ad libitum for 2 weeks. The results in adult rats clearly indicated that both rice proteins had cholesterol-lowering effects in the plasma and the liver, comparable to soy protein, and the effects were accompanied with TG-lowering effects in the liver. Similar effects were also observed in growing rats when the diets were supplemented with cholesterol. The mechanism of the cholesterol-lowering effects by these rice proteins cannot be explained solely by fecal steroid excretion, but the results indicate that not only regular rice protein but also Shunyo rice protein possesses improving effects on lipid metabolism, especially in the adult period.