The role of protein amide groups was studied in the process of aging. With aging asparagine deamidation occurs in water-soluble and water-insoluble proteins of the rat brain, liver and heart. The glutamine content in the old rat tissues remains at the level determined in the young animal tissues. Tissue proteins of old rats are better substrates for a complex of proteinases of a wide specificity than those of young animals. An increased attack of old animals proteins by nonspecific proteinases is due to a decrease in their amidation during aging.