[im-Bzl D-His.sup.6 ]LRF and [D-His.sup.6 (im-Bzl), Pro.sup.9 -NEt]LRF exhibit hydrophillicity comparable to that of LRF and act as superagonists exhibiting potencies, respectively, about 12 and more than 200 times that of LRF. The peptides or their nontoxic salts can be administered by intravenous subcutaneous, sublingual, oral, intravaginal, intranasal or rectal routes. The peptides can be used to regulate fertility in male and female mammals, including human beings.