The effects of an estrogen (Diethylstilbestrol, DES) and its interaction with follicle stimulating hormone on the ovary were studied in hypophysectomized female rats. DES stimulated increased incorporation of tritiated thymidine into granulosa cell DNA, followed by an increase in the number of granulosa cells. In addition, DES increased ovarian incorporation of tritiated FSH, an effect which was shown to be time and dose related, inhibited by unlabeled FSH, and not seen with tritiated HCG. FSH was less effective than estrogen in stimulating granulosa proliferation but appeared to stimulate antrum formation after appropriate estrogen induced maturation. When the two hormones were given sequentially, there was a marked enhancement in the number of follicles containing antra and a marked increase in ovarian weight. The possible physiologic significance of these observations is discussed. (Endocrinology90: 1492, 1972)