The mode of action of the follicle-stimulating hormone (FSH) on ovarian follicle growth was studied in hypophysectomized rats using the histologic, autoradiographic and histochemical techniques. The follicle growth was stimulated by the administration of both FSH and estrogen. The histologic finding of the follicle growth induced by the two hormones was different. Namely, after the administration of FSH, the theca layer was thick, but after the administration of estrogen, it was thin. 3H-thymidine and 3H-leucine were used to investigate cell division in a growing follicle. The uptake of 3H-thymidine and 3H-leucine by the theca layer was enhanced remarkably by FSH. On the other hand, the uptake of 3H-thymidine by the granulosa layer was enhanced by FSH or estrogen, while the grain count of granulosa cells was increased only by the administration of estrogen. Moreover, the administration of FSH resulted in an increase of the enzyme activity of glucose-6-phosphate dehydrogenase (G-6-PD), DELTA5-3beta-hydroxysteroid dehydrogenase (3beta-HSD) and alkaline phosphatase (ALPase) in the theca layer. Furthermore, the administration of FSH caused an increase in the serum estradiol and estriol of rats, whereas the administration of estrogen did not. It seems possible, therefore, that FSH stimulated proliferation of theca cells and produced estrogen. The results suggest that the estrogen produced by the theca cells might stimulate proliferation of granulosa cells; consequently, follicle growth might be induced.