During meiotic maturation, numerous cytoplasmic and nuclear events take place that prepare the oocytes for fertilization. These changes are initiated by an increase in the activity of several kinases, most notably maturation-promoting factor, also called histone H1 kinase. Another kinase, mitogen-activated protein (MAP) kinase, is also stimulated during this period. In this study, we investigated the role of MAP kinase in bovine oocyte maturation. First, the kinetics of activation of histone H1 and MAP kinases during maturation were assessed simultaneously by evaluating their catalytic activities in vitro. We found that they are activated at approximately the same time, around germinal vesicle breakdown (GVBD). Then, at approximately 15 h of maturation, the activity of H1 kinase temporarily decreases, whereas MAP kinase remains high through the metaphase II stage. Second, the activation and catalytic activity of MAP kinase was directly evaluated by Western blotting and by an in-gel kinase assay. We determined that MAP kinase becomes activated and exhibits a decreased mobility through SDS-polyacrylamide gels, and that its catalytic activity increases as maturation progresses. In our system, most of the MAP kinase activity can be attributed to p42MAPK2. Third, the activation pathway of MAP kinase was explored. In Xenopus oocytes, MAP kinase is activated by a kinase cascade that includes several upstream activators; one of them is the product of the proto-oncogene mos. In bovine oocytes, injection of Mos RNA elicited a rapid maximal activation of MAP kinase that resulted in accelerated resumption of meiosis and GVBD. These results were thought to be mediated by an expression of a kinase-active Mos RNA failed to activate MAP kinase. Together, these results suggest a role for MAP kinase during the initiation and progression of meiosis in bovine oocytes. The data also suggest the presence of an MAP kinase-activating cascade that can be initiated by the Mos protein.