Certain types of nutritional restriction can prolong life in mammals. This investigation documents life long effects of five protein diets in 1,000 Balb/c male mice. Balb mice subjected to 4% protein (lowest) diet had life expectancy that was marginally significantly prolonged when compared with control mice fed 24% protein (highest) diet. Body and organ weights of protein restricted mice generally were less than control mice. No significant differences among diets were found with blood counts, protein concentrations of liver and kidneys, or kidney catalase activity. Kidney catalase activity fell with age. Liver aldolase was induced by dietary sucrose, and aldolase fell with age in protein restricted mice, but not in controls.