Young and adult male rabbits were alternately fed semipurified diets and a commercial diet over a period of 57 weeks. The semipurified diets, containing either casein or soy protein, and the commercial diet were fed either ad libitum or on a restricted basis. When a restricted feeding regime was applied, both in the young and adult rabbits, significantly higher levels of serum cholesterol were observed in the animals fed casein compared with their counterparts fed soy protein. However, during the first period of feeding the semipurified diets, the hypercholesterolemic response of the casein diet was significantly greater in the young than in the adult rabbits. During the second and third period, no significant differences in cholesterolemic response were observed between young and adult rabbits fed casein diets. Further, the cholesterolemic response to semipurified diets containing casein was progressively lower during the second and third period. Similar results were found when the rabbits were fed ad libitum. However, in the adult rabbits no significant differences were observed between the rabbits fed casein and soy protein. Thus, the results of this study show that adult rabbits are less susceptible to the induction of hypercholesterolemia by feeding casein diets than are young ones.