To study further the role of gonadotropins in reproductive functions, we generated mice with LH receptor (LHR) knockout (LuRKO) by inactivating, through homologous recombination, exon 11 on the LHR gene. LuRKO males and females were born phenotypically normal, with testes, ovaries, and genital structures indistinguishable from their wild-type (WT) littermates. Postnatally, testicular growth and descent, and external genital and accessory sex organ maturation, were blocked in LuRKO males, and their spermatogenesis was arrested at the round spermatid stage. The number and size of Leydig cells were dramatically reduced. LuRKO females also displayed underdeveloped external genitalia and uteri postnatally, and their age of vaginal opening was delayed by 5-7 days. The (-/-) ovaries were smaller, and histological analysis revealed follicles up to the early antral stage, but no preovulatory follicles or corpora lutea. Reduced gonadal sex hormone production was found in each sex, as was also reflected by the suppressed accessory sex organ weights and elevated gonadotropin levels. Completion of meiosis of testicular germ cells in the LuRKO males differs from other hypogonadotropic/cryptorchid mouse models, suggesting a role for FSH in this process. In females, FSH appears to stimulate developing follicles from the preantral to early antral stage, and LH is the stimulus beyond this stage. Hence, in each sex, the intrauterine sex differentiation is independent of LH action, but it has a crucial role postnatally for attaining sexual maturity. The LuRKO mouse is a close phenocopy of recently characterized human patients with inactivating LHR mutations, although the lack of pseudohermaphroditism in LuRKO males suggests that the intrauterine sex differentiation in this species is not dependent on LH action.