Abstract: The concentration of sex steroids in the serum of 7 females rhesus monkeys during the intermenstrual period were measured by radioimmunoassay. Fifteen of 20 menstrual cycles have s surge of estradiol and testosterone in the middle cycles. The average surge value of estradiol and testosterone are 179.1±91.3pg/ml and 1010.1±411.3pg/ml respectively. In normal luteal phase, the value of progesterone is not below 1ng/ml. The average length of 20 menstrual cycles is 28.6±5.4 days (mean±SD). Follicle phase is 11.9±2.8 days. Luteal phase is 19.2±6.3days. The menstrual cycles are expressed by the formua Y=18.92±0.03X²(Y: menstrual cycle, X: luteal phase; R=0.9444). The results suggest that musuring the three steroids hormones can be accurately indexed ovulatory in rhesus monkeys. The mechanism of secretion and regulation of testosterone in reproductive cycle will be studied continuously.