Although cytological alterations in the hypophysis of hypo-and hyperthyroid animals have been extensively described by many investigators, electron microscopic description of the anterior pituitary in experimental hyperthyoid animals has previously been little reported. Experimental hyperthyroidism induces a quantitative alteration of various hormonal conditions in the anterior pituitary. The present auther previously, reported on the anterior pituitary cytology of the hyperthyroid mouse made with the light microscope (U chid a, '64). In the present study, light and electron microscopes were employed to investigate the cytological changes in the mouse anterior pituitary following thyroxine injection.