We have investigated follicular atresia by giving hypophysectomized immature female rats (HIFR) diethylstibestrol or gonadotrophins with and without the chemical antiestrogen CI-628, making total counts of normal and atretic follicles greater than 125 μM in diameter, and using a simple model to analyze data. Our results show an anti-atretic effect of estrogen, independent of its well documented mitogenic effect on preantral follicles. We have also shown that CI-628 acts as an anti-estrogen to block follicular proliferation, while acting as an estrogen to inhibit atresia. In addition, we have observed an increase in atresia caused by gonadotrophins, in opposition to their estrogen-mediated positive effect on follicular growth. (Endocrinology96: 1145, 1975)

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