Epithelial organs (kidneys, testis) of rats, guinea-pigs, rabbits, cats and dogs, after the blood was removed, showed a constant Fe content. In the liver the Fe level is usually a little higher than in the other epithelial organs. Within a short time the Fe content increases up to 200% and remains at this level until death. This newly acquired Fe is probably a compound other than those already present in the cells and must be associated with the aging of the cells.