The presence of elongated acidophils with a long process extended to the blood capillary was confirmed by optical and electron microscopies in the normal infantile and adult rats. They are principally localized at the periphery of the gland and characterized by the medium-sized granules of 200-250mμ in diameter sometimes along the plasmamembrane. Elongated acidophils resemble, in their cytological properties, both the ACTH secreting cells by Moriartry and Halmi (1972) and immature acidophils in postnatal rats (Watanabe 1974). In our electron microscopic observation on the normal adult rats, three kinds of central spherical acidophils are distinguished: The first type contains purely small granules 130-200mμ in diameter, the second has large and small granules ranging from 130 to 350mμ in diameter, and the third only large ones about 350mμ in diameter. The first one was first described in the present study, while so-called “somatotrophs” might belong, in our opinion, to the second or third group. The definite response to adrenalectomy appeared on a series of acidophils. After adrenalectomy elongated acidophils were deprived of medium-sized granules within 24hr and they became agranular cells, but gradually re-produced their granules with the lapse of postoperative days. Some central spherical acidophils fully stored small granules within 5 and 7 days. Consequently, the proportion of the first type of acidophils increased. On the other hand, a series of basophils also underwent some changes following adrenalectomy. Above all, some of gonadotrophs stored up granules of low density along with haloed granules regardless of the diversity in postoperative interval, despite the retrogressive sign in some other gonadotrophs. Following adrenalectomy, some thyrotrophs of the irregular shape or stellate-shape were also found to contain a limited number of haloed granules in the Golgi area. These cells may correspond with “corticotrophs” either by Kurosumi and Kobayashi (1966) or by Siperstein and Miller (1970). They, however, never hypergranulated at any postoperative periods. In conclusion, the acidophils containing only the small granules were shown to exist even in the normal adult rat, and adrenalectomy gave rise to definite release subsequently eading to storage of the small granules. The parallelism between pituitary ACTH content (Hodges and Jones, 1964) and the number of small granules may suggest some close correlation between these granules in acidophils and ACTH storage.