The effects of sex stimulating hormones on abnormal mature ova of mice were investigated cytologically during the sexual cycle. The results indicate that abnormal mature ova preceding further than the first maturation stage are very few in number in hormone treated mice than in the untreated mice. Especially, the P.M.S. treated mice indicate the remarkable decrease in number of abnormal ova. Further the decrease in number of abnormal ova showing an immature condition of granulosa cells is remarkable. It seems probable that the sex stimulating hormones are not effective on polyovular follicles. Based on the results of the present study, the assumption is possibly made that abnormal mature ova which failed to ovulate may undergo ovulation under the effect of sex stimulating hormones.