Human ovarian tissue pieces from the patients with malignant uterine tumor were studied by electron microscopy. In the process of folliculogenesis, the follicle becomes surrounded by the prospective theca interna cells and, at the tertiary stage, fibroblast-like stem cells, transitional cells and well-differentiated theca interna cells are recognized in the theca interna layer. Following follicular growth, many mitotic figures are observed in theca interna. Some consist of stem cells and others are well-differentiated theca interna cells, which reveal the cytoplasmic constituents as being complete steroidogenic cells. In the corpus luteum limited to the first half of the luteal phase, well-differentiated theca interna cells also proliferate, showing mitotic activity. Moreover, corpus luteum of pregnancy enlarges and theca interna cells proliferate under very high serum hCG level in the early stage of gestation. It may be estimated that the rapid increase of estradiol concentration in blood plasm following follicular growth and corpus luteum development is induced by the rapid increase of the number of theca interna cells, through the proliferation and differentiation of stem cells as well as the proliferation of well-differentiated theca interna cells.