Background: Many research works showed that menstruation is caused by the sudden reduction of the estrogens and progesterone to low levels of secretion at the end of the menstrual cycle. However, it should be investigated to which levels the concentration of these two hormones sharply reduces to cause menstruation. Objective: To determine estradiol concentration at the beginning of menstruation, the second day of menstruation and the ending of menstruation during the menstrual cycle. Subject and methods: 30 women with the normal menstrual cycle and 30 women with the long menstrual cycle were used as subjects to determine estradiol concentration at the day at the beginning of menstruation, the second day of menstruation and the last day of menstruation during the menstrual cycle. Result and Conclusion: In the women group with the normal menstrual cycle, the estradiol concentration: at the day at the beginning of menstruation is 159.93 \xb1 51.29 pmol/L, at the second day of menstruation is 170.58 \xb1 46.54 pmol/L and at the lastday of menstruation is 177.55 \xb1 46.42 pmol/L. In women in the group with long menstrual cycle, the estradiol concentration: at the day of beginning of menstruation is 154.55 \xb1 61.60 pmol/L and at the last day of menstruation is 154.55 \xb1 61.60 pmol/L.
menstrual cycle ; estrogen ; estradiol