Numerous studies have shown caloric restriction retards the physiological decline and increases the life span of animals. However, in these studies protein consumption was also reduced; thus, whether the beneficial effects were due to caloric or to protein restriction is unclear. To examine independently the effects of caloric and protein restriction on growth, renal function, and survival, caloric restricted male rats were fed 18, 30 or 42 percent casein diets that provided two-thirds of the quantity of diet consumed by groups fed 12, 20, or 28 percent casein diets ad libitum, respectively. Hence, caloric restricted groups consumed the same amount of protein as their paired ad libitum fed groups but one-third fewer calories. The results showed that caloric restriction decreased mature body weight, increased the rate of attaining mature body weight, retarded the age-associated decline in renal function, and increased survival. Protein restriction had no effect on mature body weight, decreased maturation rate, improved renal function, and decreased survival. Thus, protein restriction did not contribute to the survival-promoting effects of caloric restriction in rats.