Micro-injection of catalytically inactive GST-cdc2-K33R or GST-cdk2-K33R fusion proteins, each of which efficiently titrates cyclin B in oocytes and prevents assembly of cyclin B-cdc2 complexes, readily induces premature DNA replication in starfish oocytes after emission of the first polar body. Moreover, partial ablation of cyclin B mRNA by micro-injection of antisense oligonucleotides facilitates premature DNA replication induced by the dominant-negative cdc2 and cdk2 mutant proteins. We thus propose that enhanced translation of cyclin B after GVBD, a universal feature of oocyte maturation in the animal kingdom, and subsequent assembly of cyclin B-cdc2 complexes, are part of the checkpoint that prevents DNA replication in the oocyte after emission of the first polar body. MAPK inactivation is neither required for premature DNA replication after the first meiotic cell cycle nor for DNA replication after completion of meiotic maturation. However, micro-injection of a N-terminally truncated form of the budding yeast STE11 protein, that constitutively maintains MAPK active after the second meiotic cleavage, prevents fertilized eggs from proceeding into embryogenesis, and arrests them at G2, as is the case in unfertilized eggs that cannot inactivate MAPK after the second meiotic cleavage. We thus propose that MAPK functions in meiotic maturation by preventing unfertilized eggs from proceeding into parthenogenetic development.