LH and FSH act on gonadal cells to regulate steroidogenesis and gametogenesis. To model human reproductive disorders involving loss of LH function and to define LH-responsive genes, we produced knockout mice lacking the hormone-specific LHβ subunit. LHβ null mice are viable but demonstrate postnatal defects in gonadal growth and function resulting in infertility. Mutant males have decreased testes size, a block in Leydig cell differentiation, and a reduction in serum and intra-testicular testosterone levels. Furthermore, spermatogenesis is blocked at the round spermatid stage resulting in a total absence of the elongated spermatids. Mutant female mice are hypogonadal and demonstrate decreased levels of serum estradiol and progesterone. Ovarian histology reveals normal thecal layer, defects in folliculogenesis including many degenerating antral follicles and absence of corpora lutea. The defects in both sexes are not secondary to aberrant FSH regulation, since FSH levels were unaffected in null mice. Finally, the null mice can be pharmacologically rescued by exogenous hCG indicating that LH-responsiveness of the target cells is not irreversibly lost. Thus, LHβ null mice provide a useful model to study the consequences of an isolated deficiency of LH ligand in reproduction, while retaining normal LH-responsiveness in target cells.