The ability of various steroids to induce maturation of Xenopus laevis oocytes after incubation or injection has been studied. Only two of these hormones, hydrocortisone and testosterone, are active by injection as well as by incubation. Progesterone can induce maturation only when administered from the outside medium, which suggests the presence of a receptor in or near the membrane. This progesterone‐receptor has been localized in the melanosomes, and its properties and affinity for other steroids have been studied. The morphological and biochemical effects of the injection of the progesterone‐receptor complex have been studied.