Development and differentiation of rat follicular cells were investigated by lightand electronmicroscopy. The cell-cords growing out from the anterior wall of Rathke's pouch at the 15th day of gestation gradually form the original anterior lobe. Thereafter, its agranular primordial cells spread throughout the gland. Most of them begin to granulate, while some others undergo only mitotic division. The latter tends to congregate and repeat the mitosis for the appointed duration. These cells surround a pseudolumen equipped with microvilli, constituting a cell-mass lacking the basement membrane at the 17, 18 and 20th day of gestation. They may lead to the follicular cells. Pituitary marginal cells resemble in fine structure the follicular cells. The former is a residue of the anterior wall of Rathke's pouch, while the latter is a cluster of the cells differentiating for the specific purpose from the cell-cords. Even in postnatal and infant rats, clusters of ungranulated follicular cells are preserved and may serve as the proliferating center of the gland, based on the frequent observation of mitosis there. After mitotic division, the intercellular spaces enter midway between two daughter cells, leading to the formation of a cluster consisting of small irregular cells. The follicular cells permanently keep their undifferentiated capacity for mitosis through the life course of rats (6-, 21-, 45-, 90-and 400-day-old). The follicular cells located at the periphery of a cluster spontaneously begin to granulate after completing their own mitosis, and finally transform into immature acidophils or basophils through the intermediary stage of ambiguous cells. With granulation, they move towards the outside of the cluster to intermingle among the cells which already have been grown. So-called folliculo-stellate cells, making a framework in the gland, seem to be analogous with the isolated follicular cells without cluster formation. Thyroidectomy and autotransplantation of pituitaries successfully gave rise to granulation and multiplication of the follicular cells, but all our experiments failed to completely disperse them. The follicular cells have two capabilities, cell proliferation and differentiation, both of which are interpreted to be intrinsically balanced. It may be hypothesized in references to our previous accumulated data reported elsewhere that the pituitary belongs to the “renewal cell system” and the follicular cells may be regarded as the “stem cells” of the pituitary.