Mutations in the GnRH receptor gene (GNRHR) can result in hypogonadotropic hypogonadism in humans. Unlike most mammals, mice lack a second form of GnRH (GnRH2) and a type 2 GnRH receptor. To determine whether the GnRH receptor is critical at all stages of reproduction and whether this receptor has additional physiological functions in developing and adult mice, we have generated mice from an embryonic stem cell line containing a retroviral vector with multiple stop codons inserted into intron 1 of the Gnrhr gene. This gene trap insertion resulted in the disruption of exon 2 and exon 3 of the Gnrhr gene. The insertion also contained a lacZ gene that was used as a reporter for GnRH receptor expression in these mice. This model has a similar phenotype to the clinical syndrome of hypogonadotropic hypogonadism. Null Gnrhr mice had small sexual organs, low levels of FSH, LH, and steroid hormones, failure of sexual maturation, infertility, and inability to respond to exogenous GnRH. However, the defective GnRH receptor did not prevent morula/blastocyst development, implantation, masculinization of fetal male mice, or maintenance of early pregnancy. The phenotype of this null Gnrhr mouse was more severe than models in the literature, including the N-ethyl-N-nitrosourea-induced Gnrhr mutant, the kisspeptin (Kiss1) knockout, and the kisspeptin receptor (Gpr54) knockout. In terms of gonadal morphology, adult gene trap-Gnrhr null mice demonstrate a complete cessation of reproduction and serve as an important model for understanding GnRH/GnRHR physiology.