The pellets of chromophobes selectively isolated from the fresh rat pituitaries according to the procedure of Ishikawa (1969) were homotransplanted in the hypophysiotrophic area of hypophysectomized rats. Prior to transplantation, the pellets were examined histologically to confirm no contamination with cells of the other kind. In the early phase, up to 24hr. after transplantation, chromophobes proliferated through mitosis. In the middle phase, from 36hr. to 7days, they differentiated into acidophils or basophils to a degree via intermediate cells. Mitotic division which preceded differentiation led to further proliferation. In the later phase, from 10 to 30 days after transplantation, the chromophobes were completely transformed into two types of chromophils without undergoing mitosis. Thus, a direct evidence was adduced that the chromophobes were transformed into chromophils along the acidophil-and basophilaxes. From the findings on the pellets grafted beneath the renal capsule or into the cerebral cortex in controls, it was postulated that hypothalamus not only exerted a strong action on the chromophobes to make them differentiate into chromophils, but also reorganized the isolated chromophobes into the anterior pituitary.