Maturing vertebrate oocytes become arrested in metaphase of the second meiotic division. These oocytes are ovulated, and then can be activated by sperm or parthenogenetic stimulus. Metaphase arrest is mediated by the cytostatic activity CSF, that prevents the inactivation of the major M-phase regulator i.e. CDK1-cyclin B kinase. CSF inactivation seems to be necessary for the completion of the second meiotic division and the initiation of the embryonic development. Analysis of amphibian oocyte maturation led to the discovery of factors crucial for the CSF activation.