In order to quantitate the feedback effect of estradiol (E2) on the release of LH, serum LH and E2 were measured in 27 ovulatory women daily during the menstrual cycle. The initial rise of LH levels concomitantly occurred when serum E2 concentrations over 100 pg/ml and over 200 pg/ml were maintained for 2 approximately 4 days and for 1 approximately 2 days respectively. The initial rises of LH levels were observed on the day of preovulatory E2 peak. A more detail analysis of the relationship between the two hormones were obtained in five subjects whose blood samples were taken at every eight hours around the time of ovulation. In one among five cases, samples were not obtained early enough to ascertain the relation of the E2 peak to the LH peak. In 3 out of the 4 cases, the intial rises of LH levels occured prior to or concomitant with the time of the E2 peak. By average of all of the five cases, a significant initial rise of LH levels was observed simultaneously with the time of the E2 peak. These results suggest that the increase of the circulatory E2 levels is essential for the initiation of LH surge and their decrease levels are an unrequired mechanism for the midcycle LH release.