To investigate the role of mitogen-activated protein (MAP) kinase kinase (MEK)/MAP kinase cascade on p34^cdc2 kinase activity and cyclin B1 levels during parthenogenetic activation of porcine oocytes, MEK activity, MAP kinase activity, p34^cdc2 kinase activity, and cyclin B1 levels were assayed in mature porcine oocytes after treatment with different concentrations of Ca² ionophore. A high concentration of Ca² ionophore (50 μM) rapidly reduced MEK activity in oocytes for up to 8 h of culture. MEK activity in the 10-μM treatment group was significantly higher. The low concentration treatment transiently decreased p34^cdc2 kinase activity but did not affect MAP kinase activity and ultimately induced reactivation of p34^cdc2 kinase via the synthesis of cyclin B1. On the other hand, treatments of a high concentration of Ca² ionophore or a low concentration of Ca² ionophore plus MEK inhibitor, U0126, linearly decreased MAP kinase activity following the decrease of p34^cdc2 kinase activity; most of these oocytes formed pronuclei. These results suggest that decreasing MAP kinase activity is essential to maintaining low p34^cdc2 kinase activity resulting from the degradation of cyclin B via a Ca² -dependent pathway; lower activities of both MAP kinase and p34^cdc2 kinase induce normal meiotic completion and pronuclear formation of parthenogenetically activated porcine oocytes.