The gonadotropin-releasing hormone (GnRH) receptor (GPCR-A6 subfamily) is expressed on the surface of pituitary gonadotrope cells as well as lymphocytes, breast, ovary, and prostate. The fundamental role of hypothalamic GnRH in the reproductive system by stimulation of pituitary gonadotropin secretion has made it a prime drug target for treatment of infertility, sex hormone-dependent diseases and for novel contraception in man.