The occurrence of polyovular follicles (PF) was examined at 10‐34 days of age in the ovaries of BALB/cCrgl female mice given five daily injections of 0.1μg diethylstilbestrol (DES), 2μg DES, 100 μg progesterone (P), 137 μg 17α‐hydroxyprogesterone caproate (HPC), 20 μg testosterone (T), 20 μg 5α‐dihydrotestosterone (5α‐DHT), or oil vehicle alone starting on the day of birth, and of C57BL/Tw females given five neonatal injections of 1 μg DES, 20 μg 17β‐estradiol (E₂), 50 μg 5α‐DHT, 50 μg 5β‐DHT, or the vehicle alone. Ovaries of 30‐day‐old C57BL mice given five daily injections of 1 μg DES starting at 3–25 days of age were also examined. PF incidence (% of PF per ovary) and PF frequency (% of mice with PF) were significantly greater in BALB/c mice receiving injections of DES, P, HPC, and T than in the controls. In DES‐treated mice at 34 days, PF incidence (2–13 oocytes/follicle) was 120–340 times higher than in the controls. BALB/c mice treated with T, P, and HPC showed PF incidence (two to four occytes/follicle) three‐ to six‐fold higher than in the controls. In 30‐day‐old C57BL mice treated with T, E₂, and DES, PF incidence also increased by two‐ to 50‐fold. 5α‐DHT and 5β‐DHT failed to increase PF incidence. PF incidence was significantly increased only when neonatal DES treatment was begun on days 0 to 3, but was reduced when started at days 10–25. In DES‐treated C57BL mice aged 10–30 days, PF incidence was significantly higher than in the age‐matched controls.