Abstract
The enlargement of the pituitary gland of the mouse treated with I131 was confirmed. This enlargement is limited to the anterior lobe. Microscopic examination of the anterior lobe revealed a benign hypertrophy and hyperplasia of the basophilic cells and degranulation of the acidophils. No cytopathological alterations were observed in the posterior or intermediate lobes. The hypertrophy and cytological changes induced in the anterior lobe by the I131 injns. were prevented by the feeding of desiccated thyroid. It is concluded that the enlargement resulted from the hypothyroidism which followed the destruction of the thyroid by the ionizing radiation.