In 35-day-old female ICR/JCL mice given 5 daily injections of 1 microgram diethylstilbestrol (DES) from the day of birth, a significantly higher incidence of polyovular follicles was found in the ovaries than in those of age-matched control mice. Gap junctions of granulosa cells of mature follicles in neonatally DES-exposed mice were larger than those of the controls. When stimulated by gonadotropins (PMSG and hCG) and caged with males, the number of tubal embryos in DES-exposed mice was less, but the rate of fertilization and development was not different compared to the controls. Division of oocytes collected from the ovaries of 40-day-old DES-exposed and control mice after stimulation of gonadotropins was examined 24 to 72 h after in vitro insemination to ascertain whether fertilization had occurred in oocytes from polyovular follicles. Seventy-seven % of oocytes from uniovular follicles of control mice developed up to 8-cell stage embryos following in vitro insemination; 66% of those from similar follicles of DES-exposed mice developed into the same stage. By contrast, only 47% of oocytes from polyovular follicles of DES-exposed mice showed the division up to 8-cell stage 72 h after insemination, indicating a significantly lower fertilization rate compared to the oocytes from uniovular follicles of control and DES-exposed mice. Without insemination, oocytes taken from the same pools in these experiments never divided during the period of manipulation and incubation.(ABSTRACT TRUNCATED AT 250 WORDS)