Fully grown Xenopus oocytes can remain in their immature state essentially indefinitely, or, in response to the steroid hormone progesterone, can be induced to develop into fertilizable eggs. This process is termed oocyte maturation. Oocyte maturation is initiated by a novel plasma membrane steroid hormone receptor. Progesterone brings about inhibition of adenylate cyclase and activation of the Mos/MEK1/p42 MAP kinase cascade, which ultimately brings about the activation of the universal M phase trigger Cdc2/cyclin B. Oocyte maturation provides an interesting example of how signaling cascades entrain the cell cycle clock to environmental changes. Copyright 1999 John Wiley & Sons, Inc.