Although the role of oxidative stress in maternal aging and infertility has been suggested, the underlying mechanisms are not fully understood. The present study is designed to determine the relationship between mitochondrial function and spindle stability in metaphase II (MII) oocytes under oxidative stress. MII mouse oocytes were treated with H 2 O 2 in the presence or absence of permeability transition pores (PTPs) blockers cyclosporin A (CsA). In addition, antioxidant N-acetylcysteine (NAC), F 0/F 1 synthase inhibitor oligomycin A, the mitochondria uncoupler carbonyl cyanide 4-trifluoro-methoxyphenylhydrazone (FCCP) or thapsigargin plus 2.5 mM Ca 2+(Th+ 2.5 mM Ca 2+) were used in mechanistic studies. Morphologic analyses of oocyte spindles and chromosomes were performed and mitochondrial membrane potential (ΔΨm), cytoplasmic free calcium concentration ([Ca 2+] c) and cytoplasmic ATP content within oocytes were also assayed. In a time-and H 2 O 2 dose-dependent manner, disruption of meiotic spindles was found after oocytes were treated with H 2 O 2, which was prevented by pre-treatment with NAC. Administration of H 2 O 2 led to a dissipation of ΔΨm, an increase in [Ca 2+] c and a decrease in cytoplasmic ATP levels. These detrimental responses of oocytes to H 2 O 2 treatment could be blocked by pre-incubation with CsA. Similar to H 2 O 2, both oligomycin A and FCCP dissipated ΔΨm, decreased cytoplasmic ATP contents and disassembled MII oocyte spindles, while high [Ca 2+] c alone had no effects on spindle morphology. In conclusion, the decrease in mitochondria-derived ATP during oxidative stress may cause a disassembly of mouse MII oocyte spindles, presumably due to the opening of the mitochondrial PTPs.