The author, using 235 hamsters of both sexes of various ages raging from birth to 300 days, made the histological and histogenetic study of the pars intermedia of the hypophysis of the hamsters. The results obtained were as follows1. The pars intermedia is composed of epithelial parenchymatous cells, border layer cells which lie along the hypophyseal cleft, connective tissues and very few blood vessels, but do not contain nerve fibers.2. In the intermedis cells, the mitoses are most numerous in number at the 3th day, and always found throughout life, although they are rare in adult.3. In adult the parenchymatoua cells are large and shaped variously. The cytoplasm is abundant and granular, staining basophilic, and the cell boundaries are not always clear. The parenchymatous cells are stained positively with GOMORI's aldehyde-fuchsin and periodic acid-SCHIFF reaction. The nuclei, irregularly shaped, generally contain one or two acidophilic nucleoli.4. The author confirms that nuclear spheres derived from nucleoli, discharge the contents, and thus that the nuclei become irregularly shaped, showing incisures and folds.5. In earlier stages of life, the parenchymatous cells are grouped very densely, and then they become larger. At the 30th day the nuclear sphere first appears, and at the 40th day a small number of cells show the nuclei with incisures and folds.6. In adult, the border layer cells have scanty cytoplasm with ovoid, cylindrical or flatt nuclei. These cells are seen at 7th day for the first time and then gradually increase in number. These cells differenciate from the parenchymatous cells.7. The connective tissues are scanty in the pars intermedia.