Effects of the isoflavone supplement on hormonal states in young premenopausal women were studied by cross-over study design. Administration of 20 mg or 40 mg isoflavones (IF) by tablets, of which 1 g contained 43.5 mg daidzein, 6.0 mg genistein, 24.0 mg glycitein, to 40 young female students for one month caused a prolonged menstruation in 60% of young women, shortened menstruation in 20% of young women, 17% remained unchanged and 3% became irregular. Larger dose tended to elongate more, but 17beta-estradiol levels in both follicular and luteal phages were not different between 20 mg and 40 mg isoflavone intake. Equor excreters tended to show low plasma progesteron level in the luteal phase. Detailed hormonal analysis on 3 of students by a cross over study design showed decreased level of 17beta-estradiol throughout the menstruation cycle. SHBG significantly increased about 10% in all three. DEAS, androstendione, and testosterone showed different responses according to the follicular or luteal phase. T3 and T4 increased as a result of isoflavone tablet administration in the follicular phase, but it decreased in the luteal phase. These changes suggest that isoflavones influence not only estrogen receptor-related functions but the hypothalamo-hypophysis-gonadal axis.