Fully grown mammalian oocytes arrested at prophase I resume meiosis after gonadotropic stimulation. Oocytes undergo a series of changes including chromosome condensation, nucleolus disassembly, germinal vesicle breakdown (GVBD), and spindle formation. The mouse is the best model for studying the molecular mechanisms underlying the maturation of mammalian oocytes. However, some of the maturational events are different from those in other mammalian species. To study these events, we use pig ovaries, which are available as a byproduct from local slaughterhouses. It has long been known that pig oocytes have a dependence on de novo protein synthesis for GVBD, whereas GVBD in mouse oocytes occurs independently of protein synthesis. The reason seems to be the lack of Cyclin B1 molecules in pig GV-oocytes, although the synthesis of other protein(s) may be required for the GVBD. In mouse oocytes, the spindle is formed through the action of cytoplasmic microtubule organizing centers (MTOCs), and the oocytes are able to form the spindle without chromosomes. However, pig oocytes don't have such distinguished cytoplasmic MTOCs and never form the spindle without chromosomes. In this species, the condensing chromosome plays the role of the organizer nucleating spindle microtubules. We should develop some other mammalian models that will help us understand the mechanisms underlying oocyte maturation.