The pituitary glands of castrated male rats of Wistar strain were cytologically investigated. The animals were devided into four experimental groups: Intact control rats (1st group) aging 24, 38, 44, 52, 58, 72, 84, 98, and 112 days; gonadectomized rats at the 10th (2nd), 30th (3rd) and 70th (4th) postnatal day respectively were decapitated 2, 4 and 6 weeks after the operation. The obtained cytological observations were as follows:In control rats aging 30 days, α-cells were yet indifferentiated because of small size and deficient mitochondria and of scattered distribution in every portion of the whole anterior lobe. Only a small number of β-cells strongly stained with PAS were recognized, presenting the ordinary polygonal cell-outline. Above two chromophile cells would be immature in nature, whereas there were found numerous transparent γ-cells providing the enlarged cytoplasm and distinct cell-contour. In rats aging 44-52 days, β-cells increased both in number and size to be transformed into the spherical cell type which include so-called signet-ring (negative shadow of GOLGI apparatus) in the cell-body faintly stained with PAS. In 60-90 days, β-cells were more augemented in number consisting principally of spherical cells rich in mitochondria, whose signet-ring was typical to be easily detectable. 90-114 days group showed the similar changes in the cellular elements of the anterior lobe of pituitary gland. In our observation it was determined that chromophile cells have grown up till 80 or 90th postnatal day.However, in castrated rats at the 10th day, β-cells were multiplied and hypertrophied with the consequent self-formation into spherical or ellipsoid cell whose cytoplasm containing abundant mitochondria were reduced in staining ability. The β-cells were not located at a peculiar limited place, but they arised irregularly at the various portions in the anterior lobe. The constantly demonstrated signet-ring in β-cells apparently increased in diameter. α-cells were not only diminished in number but also somewhat atrophic with scanty mitochondria, and it may be responsible for the transformation of γ-cells into β-cells that the former was too dispersed to be frequently detectable. Also within 6 weeks, β-cells were so hypertrophied that all of them were inherent in the nature of spherical cell with typical signetring, whereas α-cells more atrophic decreased in number, and γ-cells were generally scanty.With reference to rats gonadectomized at the 30th postnatal days, even within two weeks pituitary gland cells never showed any deformations. However, within 4 weeks it was a common observation in β-cell that their enlarge spherical cytoplasm stained faintly and diffusively with PAS enclosed large signet-ring, and these alterations were profound within 6 weeks with the hyperamic phenomena in the whole anterior lobe, but insignificant in comparision with those recognized in β-cells of 10th postnatal day castrated rats.In rats castrated 70th day, β-cells swelled in some degree, but less seriously than in above two experimental groups, with an exception of seldom occurrence of expanding β-cells accumulated in the vicinity of pars intermedia or center of the anterior lobe. Within 4 weeks a plenty of hypertrophied β-cells usually enclosing the signet-ring also appeared, although they were inferior, both in the frequency of occurrence and degree of enlargement of cell-body, to those in the 30th day castrated group. Within 4 weeks a lot of polygonal β-cells were present together with a few remaining ellipsoid or spherical cells, and further many of these cells fell into a hypochromatic degeneration 6 weeks after the castration.