Nonpeptide antagonists of the human gonadotropin-releasing hormone receptor (GnRH-R) have been the subject of considerable interest because of their potential as a new class of oral therapeutics for the treatment of sex hormone-dependent diseases and infertility. While many classes of competitive GnRH-R antagonists have been described, we present here the first characterization of an allosteric nonpeptide GnRH-R antagonist. Previously, 5-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-ylmethyl)furan-2-carboxylic acid (2,4,6-trimethoxyphenyl)amide (here called Furan-1) had been demonstrated to be a potent GnRH-R antagonist both in vitro and in vivo. Using mutagenesis, the binding sites for Furan-1 and another potent nonpeptide antagonist (NBI-42902) have been mapped and are shown to be adjacent but nonoverlapping. Furan-1 is shown to affect the binding kinetics of radiolabeled peptide agonists as well as radiolabeled NBI-42902, and the kinetic data fit the allosteric ternary complex model. Furan-1 is considerably negatively cooperative with the nonpeptide antagonist and extremely negatively cooperative with the peptide agonist [125I-His5,d-Tyr6]GnRH so that it is nearly indistinguishable from an orthosteric competitive compound. Taken together, these data were used to develop a model of the nonpeptides bound to the GnRH-R binding site consistent with the current data.