We have investigated folluclar 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 muM in diameter, and using a simple model to analyze data. Our results show an antiatretic 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.