The plasma concentration of immunoreactive LRH, LS, FSH, oestradiol and progesterone were measured dialy by a sensitive double antibody radioimmunoassay during 12 cycles of 8 normal cyclic women. The mean (+/- SE) immunoreactive LRH levels in the follicular and luteal phase except the immunoreactive LRH peaks during normal cycles were 4.18 +/- 0.38 pg/ml and 4.50 +/- 0.45 mg/ml, respectively. The immunoreactive LRH peaks were observed in 11 of 12 cycles, appearing on day -4 to -1 from the LH surge in 9 cycles and on day +1 and +2 in 2 cycles. The mean value of immunoreactice LRH peaks was 42.0 +/- 11.4 pg/ml with range of 12 to 154 pg/ml. The immunoreactive LRH peak lasted for one day in 10 cycles and for 4 days in one cycle. The immunoreactive LRH peaks in different cycles of the same women did not occur on the same day relative to the LH peak. The plasma immunoreactive LRH levels measured every 10 min for 40 min periods every day in normal cyclic women during the ovulatory phase showed slight, but not significant fluctuations. Plasma oestradiol levels began to increase on day -6, reaching a peak on day -1, and were followed by peaks of LH and FSH. These data indicate that increase in serum oestradiol was followed by release of LRH from the hypothalamus and pre-ovulatory discharge of gonadotrophins from the pituitary.