The mammalian GnRH receptor is an atypical G protein-coupled receptor which lacks the C-terminal cytoplasmic tail that is present in all other seven-transmembrane domain receptors. The mouse and rat GnRH receptors contain 327 amino acids, whereas human, sheep, and bovine receptors have an additional residue in the second extracellular loop at position 191. Another notable species difference is that human receptors undergo agonist-induced internalization much more rapidly than the mouse receptor. In this report, the role of the additional amino acid (Lys191) in GnRH receptor function was studied in transiently expressed mutant and wild-type human and mouse GnRH receptors. Deletion of Lys191 from the human GnRH receptor caused a 4-fold increase in receptor expression in COS-1 and HEK 293 cells and a modest increase in binding affinity. The magnitude of the agonist-induced inositol phosphate response mediated by the deltaK191 human receptor was similar to that of the wild-type receptor, but the EC50 was decreased by about 5-fold. In addition, the rate of internalization of the deltaK191 human receptor was significantly reduced and was similar to that of the mouse receptor. In contrast to these effects of deletion of Lys191, its replacement by Arg, Glu, Gln, or Ala caused no significant change in receptor expression or function. These findings demonstrate that a specific residue in the extracellular region of the human GnRH receptor is a significant determinant of receptor expression, agonist-induced activation, and internalization.