Comparative studies were carried out under light, fluorescence- and electron-microscopic observations between young, adult and aged groups of albino rats with the following results: the neuronal packing density of locus ceruleus decreases with advanced age. In addition to the senescent loss of nerve cells, the surviving ones showed some shrinkage of perikarya and reduction or loss of Nissl substance. Some of them presented pyknosis, chromatolysis and loss of RNA in cytoplasm. Glial cells proliferated around the degenerating cells. Senescent cells showed an increase of heterochromatin and decrease of euchromatin. Changes in cytoplasmic organelles included the decrease of ribosomes, alterations in mitochondria and increase of lipofuscins. These senescent changes were thought to affect the absorption of nutrition, message exchange, protein synthesis, supply of energy and transport of materials by the nerve cells.