Abstract: Objective To observe the changes of estrogen and progestogen during the menstrual cycle in normal female cynomolgus monkeys. Methods Twenty sexually mature female cynomolgus monkeys aged 4-6 years and weighed 3.4-8.4 kg were enrolled in this study. Their blood specimens were collected regularly every day for 31 ?51 days to determine the serum levels of estradiol (E2) and progesterone (P) by microparticle enzyme immunoassay. Besides，the changes of E2 and P during the menstrual period，follicular phase, ovulatory period and luteal phase were observed in 6 of these monkeys with distinct menstrual cycle. Results The serum levels of E2 and P changed along with the menstrual cycle. The peak values of E2 occurred during the ovulatory period as well as luteal phase with the highest value during the ovulatory period; the peak values of P occurred during the luteal phase. Conclusion The changes of estrogen and progestogen during the menstrual cycle in normal female cynomolgus monkeys are similar to those in women of childbearing age. Therefore, normal female cynomolgus monkeys may be taken as models to study the changes of estrogen and progestogen levels in various stages of menstrual cycle in women.

Key words: Cynomolgus monkeys, Menstrual cycle, Estrogen, Progestogen