The decrease in maturation-promoting factor (MPF) activity precedes that inmitogen-activated protein kinase (MAPK) activity after egg activation, butthe cellular functions of this delayed inactivation of MAPK are stillunclear. The present study was conducted to examine the essential role ofMAPK activity for supporting the transition from metaphase to interphasein porcine oocytes matured in vitro. The increases in the phosphorylatedforms of MAPK and the activities of MAPK and histone H1 kinase (H1K) wereshown in oocytes arrested at the metaphase II (MII) stage. After additionalincubation of MII-arrested oocytes in medium with added U0126, a specificinhibitor of MAPK kinase, 24% of oocytes completed the second meioticdivision and underwent entry into interphase with pronucleus (PN) formation,but not second polar body (PB-2) emission. The intensities of thephosphorylated forms of MAPK and the activities of MAPK and H1K inmatured oocytes treated with U0126 were significantly decreased by thetreatment with U0126. Electrostimulation to induce artificial activationcaused both H1K and MAPK inactivation; the inactivation of H1K precededthe inactivation of MAPK and sustained high levels of MAPK activity weredetected during the period of PB-2 emission. However, the time sequencerequired for MAPK inactivation was significantly reduced by the additionof U0126 to the culture medium following electrostimulation, resulting inthe dramatic inactivation of MAPK distinct from that of H1K. In theseoocytes, PB-2 emission was markedly inhibited but little difference wasfound in the time course of PN formation compared with oocytes not treatedwith U0126. These findings suggest that the decrease in MAPK activity ispartly involved in driving matured oocytes out of metaphase to induce PNdevelopment, and that the delayed MAPK inactivation after the onset of MPFinactivation in activated oocytes has a crucial role for PB-2 emission toaccomplish the transition from meiosis to mitosis.