There are three possibilities concerning the mechanism of increase in number of gonadotrophs in the anterior pituitary after castration: one is the cell division by mitosis of matured gonadotrophs, another is the differentiation of immature cells, and the third is conversion of some other cell types into gonadotrophs. In order to determine the above possibilities, light and electron microscopic examinations using a combination of immunocytochemistry and autoradiography were performed. Normal and castrated adult male rats were used. Morphometry indicated that a highly increased population of gonadotrophs reacted with anti-LH beta serum after castration. Increased numbers of mitoses and elevated incorporation of 3H-thymidine into the nuclear chromatin were observed in both anti-LH beta reactive cells and other pituitary cells after castration. Electron microscopic immunocytochemical observations demonstrated the presence of immature cells with a few immunoreactive secretory granules in the castrated pituitary. It is considered that mitosis is a major factor for proliferation of gonadotrophs in the anterior pituitary after castration, but the possibility of differentiation from immature cells into gonadotrophs cannot be ruled out. The third possibility could not be proven in this study.