Control of oocyte aging during manipulation of matured oocytes should have advantages for recently developed reproductive technologies, such as cloning after nuclear transfer. We have shown that the enhanced activation ability and fragmentation of porcine in vitro matured and aged oocytes bore a close relationship to the gradual decrease in maturation/M-phase promoting factor (MPF) activity and that porcine aged oocytes contained plenty of MPF, but it was in an inactive form, pre-MPF, as a result of phosphorylation of its catalytic subunit p34(cdc2) and, therefore, had low MPF activity. We incubated porcine oocytes with vanadate and caffeine, which affected the phosphorylation status and MPF activity, and evaluated their activation abilities and fragmentation frequencies. Incubation of nonaged oocytes with vanadate increased p34(cdc2) phosphorylation and reduced MPF activity to levels similar to those of aged oocytes and increased their parthenogenetic activation and fragmentation rates compared with those of the control oocytes. Conversely, treating aged oocytes with caffeine reduced p34(cdc2) phosphorylation and increased MPF activity. These oocytes showed significantly lower parthenogenetic activation and fragmentation rates than aged mature oocytes. These results suggest that MPF activity is a key mechanism of oocyte aging and controlling MPF activity by altering p34(cdc2) phosphorylation with these chemicals may enable oocyte aging to be manipulated in vitro. We expect those ideas will be applied practically to pig cloning.