The proteasome inhibitor MG132 blocked the exit of maturing pig oocytes from metaphase I stage. The blocking effect was experienced only when oocytes were exposed to an inhibitor before 18th hour ofin vitro culture. The eefects of MG132 are fully reversible. However, a significant portion of oocytes (46%) cultured for 48 hours in MG132-supplemented medium and then for 24 hour in MG132-free medium did not block meiosis at the stage of metaphase II and underwent spontaneous parthenogenetic activation.