Synthetic LRH (Hoe 471) was injected i.v. into randomly selected eumenorrheic women. Low doses ranging from 0.1 to 1.5 /ug did not alter basal levels of radioimmunoassayable plasma LH and FSH. The minimum effective dose was 3.0 /ug LRH. Doses from 3–24 /ug LRH resulted in LH release in 14 patients, only 10 of whom reacted with FSH release as well. Maximal LH concentrations were recorded after an average 21 min following LRH injection. LH release resulting from 6, 12 and 24 /ug appears to be dose related with maximum responses of up to 110 mIU LH/ml plasma. Repeated injections of 24 /ug LRH as well as descending doses of 12, 6 and 3 /ug do not indicate any altered sensitivity to LRH of the human pituitary. LRH induced FSH release as compared to that of LH is less pronounced and defined. Ovulatory levels of LH and FSH can be maintained by continuous infusion of LRH leading to morphogenetic changes in the human ovary as shown by laparoscopy. These data are part of a series of physiological studies in the human with the aim of developing a hypothalamo-pituitary function test.