Enzyme changes in various organs of ageing mammals have been discussed, with particular reference to the limitations of the methods employed. Many enzymes have been reported to decrease, increase and remain constant in ageing, but many discrepancies exist. It was concluded that very complex patterns of change in enzymes with age exist, that combined biochemical and histochemical studies were desirable and that the old may represent a physiological elite. Bearing these limitations in mind, there may be a tendency towards a general decrease in mitochondrial respiratory enzymes in senescence.