In mice, neonatal exposure to a synthetic estrogen, diethylstilbestrol (DES), induces polyovular follicles, which contain two or more oocytes per ovarian follicle. We reported previously that the estrogen receptor beta (ESR2) mediates DES signaling in polyovular follicle induction. However, the specific mechanism of polyovular follicle induction has not yet been clarified. Folliculogenesis in rodents begins soon after birth, accompanied by programmed oocyte death and germ cell loss. In this study, the effects of DES on oocyte death and on mRNA expression of genes thought to be involved in polyovular follicle induction were analyzed during a crucial period of folliculogenesis in the ovary of C57BL/6J, Fas^lpr/lpr (lacking cell death receptor, FAS), and Esr2 knockout (Esr2 KO) mice. Neonatal DES exposure reduced programmed oocyte death in C57BL/6J mice; however, this reduction was not observed in Esr2 KO mice. In control Fas^lpr/lpr mice, the oocyte apoptotic index was significantly lower than that in the control C57BL/6J mice. However, the polyovular follicle incidence in control 20-day-old Fas^lpr/lpr mice was similar to that in the control C57BL/6J mice. Moreover, DES exposure changed mRNA expression of inhibin-alpha (Inha) in 2-day-old C57BL/6J mice. These results suggest that inhibition of oocyte death by DES through ESR2 may be one of the triggers for polyovular follicle induction. The FAS system is also involved in neonatal oocyte death; however, reduction of oocyte death is not sufficient for polyovular follicle induction. The combination of increased Inha mRNA and reduction of oocyte death in the ovaries of mice by DES through ESR2 might be correlated with polyovular follicle induction.