The effect of follicle-stimulating hormone (FSH) and diethylstilbestrol (DES) on ovarian follicle growth was studied in hypophysectomized rats, using histologic, autoradiographic and histochemical techniques. The administration of FSH to immature hypophysectomized rats stimulated the follicle growth with thickening of the theca layer and repair of "deficiency cells". Although DES given to hypophysectomized rats also stimulated the follicle growth, the theca layer was relatively thin compared with that in FSH-treated rats. In order to detect cell division in a growing follicle, the number of labelled granulosa cells with tritiated thymidine as a proportion of the granulosa cells in a growing follicle were counted and the labelling indices of the granulosa cells were calculated. The labelling indices increased by the administration of FSH (p less than 0.05) or DES (p less than 0.05). The uptake of tritiated thymidine and leucine by the theca cells was enhanced only by FSH and was not stimulated by DES. The administration of FSH resulted in an increase of the histochemically demonstrable enzyme activity such as delta5-3beta-hydroxysteroid dehydrogenase (3beta-HSD) or alkaline phosphatase in the theca cells. In contrast, the administration of DES did not stimulate the enzyme activity of the theca cells. Furthermore, FSH stimulated a release of estradiol and estriol from the ovary of hypophysectomized rat, whereas DES did not. Although ovarian follicle growth was stimulated by both FSH and DES, the effect of the two hormones on the theca cells was quite different and the secretion of estrogen was stimulated by FSH. The results suggest that FSH induced follicle growth of the ovary might be mediated by estrogen produced by the theca cells.