Ten ovulating women were studied to assess the interrelationships of various systemic, hormonal, and reproductive tract changes during normal menstrual cycles. Samples of blood, 24 hour urine specimens, vaginal smears, and cervical mucus were obtained every 2 days pre- and post-menstrually and daily in mid-cycle. Endometrial biopsies were performed at the onset of or immediately before menstruation, and the basal body temperature was recorded. Data for 10 cycles were fed into a computer, and mean reciprocal relationships were determined. The results were: (1) All endometrial biopsies reflected the late secretory phase; (2) there was a simultaneous mid-cycle surge of luteinizing hormone (LH) and folliclestimulating hormone (FSH), and serum concentrations of FSH and LH were significantly lower in the luteal phase compared to the follicular phase; (3) urinary peaks of estrone, estradiol and total estrogens occurred on the day before and that of estriol occurred on the day of the LH surge; (4) serum progesterone began to rise just before the LH peak, reached a high level 7 days after the LH peak, declined precipitately on Day 9, and rose again on Day 10, to decrease slowly until the onset of menstruation; (5) a significant relationship was observed between the LH peak and the basal body temperature; (6) urinary pregnanediol levels closely paralleled serum progesterone concentrations; (7) vaginal cytology revealed a karyopyknotic index peak the day following the LH peak; (8) properties of cervical mucus showed a remarkable relationship to the ovulatory estrogen peak.