Plasma luteinizing hormone (LH) concentrations and the numerical density (Nv) of gonadal LH-binding (LH-positive) cells were determined in intact male and female chick embryos from Days 10.5 through 18.5 of incubation for plasma LH and from Days 6.5 through 19.5 for LH-binding cells, as well as in Day 15.5 decapitated ("hypophysectomized") embryos with pituitary transplants for both plasma LH and LH-binding cells. The data demonstrate that LH is already present in the plasma of male and female embryos as early as Day 10.5, the first day examined. Plasma LH levels in hypophysectomized embryos were statistically significantly lower than those of intact embryos, while pituitary transplants to the chorioallantoic membrane of hypophysectomized embryos elevated plasma LH concentrations to levels not statistically different from those of intacts. The "role(s)" of plasma LH levels and changes in form and numerical density (Nv) of gonadal LH-binding cells in the maturation of the pituitary-gonadal axes of the chick embryo are discussed and evaluated.