Mammalian oocytes are arrested at the G2-M phase transition of the first meiotic division. In vitro, fully grown oocytes liberated from their follicles spontaneously reinitiate meiosis I, characterized by germinal vesicle breakdown (GVBD), chromatin condensation, spindle assembly, emission of the first polar body and progression to metaphase of the second meiotic division (MII), at which stage they undergo a second arrest until fertilization. After spermatozoa penetrate the oocyte, the second polar body extrudes, male and female pronuclei form and syngamy occurs to start early embryo development. Nuclear changes during oocyte maturation and fertilization are co-ordinated with movements of genetic material and organelles, and with biochemical changes in the cytoplasm to ensure normal embryo development. The normality of early embryogenesis is directly related to the ordered expression of these developmental programmes (Van Blerkom, 1991).Of the numerous cytoplasmic changes that occur, the positioning of mitochondria may be involved in concentrating ATP or calcium to specific regions in oocytes or fertilized eggs to support normal developmental processes. Thus, the distribution of active mitochondria may be indicative of the energy or ion requirement of various key events during oocyte maturation, fertilization and early embryo development. In mice, the perinuclear accumulation of mitochondria between GVBD and metaphase I (MI) (Van Blerkom and Runner, 1984; Van Blerkom, 1991), and the polarized distribution of mitochondria to one half of the oocyte containing the MII spindle (Calarco, 1995) were observed and were regarded as one aspect of the developmental programme of cytoplasmic maturation. Previous observations also revealed that translocation of mitochondria is co-ordinated

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