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. Among them are proteins involved in MAP kinase (ERK1/ERK2) pathway. Moreover, several studies focus on the factors regulating the function of the spindle assembly checkpoint and also members of the Emi protein family. Current review focus on mouse as a model organism. We discuss current understanding of the nature of cytostatic activity, its role in the meiotic maturation of mammalian oocytes and also present perspectives of further investigations.