The ovarian response to stilbestrol and to Armour''s pituitary gonadotrophin (LH) with or without stilbestrol priming was studied in both intact and hypophysectomized immature rats. In the doses used (1 mg. daily for two days) stilbestrol caused an increase in ovarian weight in intact rats and retarded the atrophy after hypophysectomy. Stilbestrol priming markedly augmented the ovarian response to gonadotrophin (LH) in both intact and hypophysectomized rats. Using a consistent dose of LH (0.5 mg.) but priming doses of stilbestrol ranging from 0.002 mg. to 2.0 mg. showed that the larger doses of stilbestrol augmented the ovarian response. Stilbestrol augments the ovarian response by making many follicles responsive to gonadotrophin. The similarity of responses in intact and hypophysectomized rats indicates that large doses of stilbestrol directly influence the ovary.