The present report by Dr. Lowry, Dr. Hastings, Dr. McCay and Miss Brown follows others made on the histochemical changes associated with aging in the muscle of the rat.In the present investigation, chemical analyses of the liver, brain, and kidney were performed for sodium, calcium, magnesium, etc.And calculations of total intracellular and extracellular material were made.Little or no change was observed in the composition of the liver and brain with senescence.However, senescence produced in the kidney a conspicuous decrease in the relative mass of the cells.