The catalytic activity of liver fructose-biphosphate aldolase (EC 4.1.2.13) of young and old rats in units per manomole sequence has been calculated. No evidence of the accumulation with age of altered enzyme molecules of low catalytic activity was obtained. This is contrary to results obtained using mice and rabbits and indicates that the accumulation of altered enzymes may not always be associated with aging. The possibility that altered proteins are formed, but do not accumulate, cannot be ruled out.