Progesterone reinitiates meiotic maturation in Xenopus laevis oocytes. Evidence is reported which indicates that the steroid acts at the level of the cell surface and suggests that an induced change of Ca²⁺ distribution triggers in turn a cascade of cytoplasmic events including protein synthesis and germinal vesicle (nucleus) breakdown. These novel features of steroid hormone action in amphibian oocytes are discussed in relation to presently accepted views of the mechanism of action of steroid hormones in somatic cells.