In order to investigate a physiological role of 17alpha-hydroxyprogestone (17-OHP) during the periovulatory phase, serum levels of LH, FSH, estradiol (E2), progesterone (P) and 17-OHP were determined by the radioimmunoassay daily throught the menstrual cycle and during the treatment for induction of ovulation. In 5 ovulatory women serum, values of these hormones were also examined every 8 hr during the periovulatory phase. By average data on five cases whose blood samples were taken every eight hr, the first significant rise in 17-OHP began 8 hr after the time of the E2 peak and the initial rise of LH levels, and the 17-OHP levels increased steeply as E2 falled down from the peak. But in 2 cases among 4, 17-OHP values increased 8 hr prior to or simultaneously with the initial rise of LH and the remaining 2 cases, 17-OHP values started to increase on 16 hr after the initial rose of LH. These results suggest that the mid cycle 17-OHP peak is not a reflection of increased E2 synthesis but that 17-OHP may have a physiological role in inducing and/or facilitating LH release. But this role of 17-OHP in LH release should be further investigated.