AbstractIn hamsters hypophysectomized on day 1 of the cycle (morning of ovulation) on days 1–28 post‐hypophysectomy 99% of follicular development is limited to small and medium sized follicles with no more than five layers of granulosal cells. Since the largest preantral follicles on day 1 of the cycle have 10–12 layers of granulosal cells, it is evident that follicles become dependent on gonadotropins at an early stage of their maturation.The daily injection of 200 μg ovine follicle stimulating hormone (FSH) on days 8–11 post‐hypophysectomy results in large antral follicles; when 10 μg ovine luteinizing hormone (LH) is then injected the animals ovulate 32 eggs. When 200 μg FSH on day 8 post‐hypophysectomy is followed by 50 μg FSH on days 9–11, injecting LH results in the ovulation of nine eggs — comparable to the ovulation rate of intact hamsters. These experiments indicate that superovulation depends on sustained high levels of FSH whereas the ovulation of a normal number of eggs requires an initially high level of FSH and then much lower maintenance levels.When replacement therapy with 200 μg FSH for four days is started on the day of hypophysectomy (day 1) or day 2, follicles fail to develop past the six‐seven granulosal layer stage. However, deferring the initial injection of FSH until day 3 results in antral follicles that are ovulated by LH (ovulation = 36 eggs). This suggests that functional corpora lutea inhibit the effects of FSH on follicular growth. Progesterone administered to hypophysectomized hamsters (days 8–11 post‐hypophysectomy) increases the percentage of medium follicles at the expense of smaller stages. Progesterone injected daily along with FSH also prevents follicular development past the six‐seven granulosal layer stage in 14 of