The adenohypophyses of adult male C,,BL/6J mice pituitaries injected with cyproterone acetate for four weeks were examined with the electron microscope. The gonadotrophs showed an accumulation of the hormone-containing granules, and a noticeable decrease in the amount of rough endoplasmic reticulum. The somatotrophs exhibited proliferated profiles of rough endoplasmic reticulum, hypertrophied Golgi apparatus, and a slight decrease in the GH-containing granules. It is believed that the cyproterone acetate may have inhibited synthesis and release of the gonadotropins, and may have stimulated the GH-producing cells through the progestational activity of this antiandrogen.