|Before fertilization, mammalian oocytes undergo meiotic maturation, which consists f nuclear and cytoplasmic differentiation. In this study, changes of Ca2?stores in ouse oocytes were examined during meiotic maturation and the role of Ca2+ in the gulation of the maturation was investigated by using monoclonal antibodies against smooth endoplasmic reticulum CaZ+-ATPase(SERCA-ATPase) and calreticulin. Observations were made under epifluorescence microscope and /or confocal laser scanning microscope. In immature oocytes which did not resume meiotic maturation, SERCA-ATPases were mostly localized in the vicinity of the germinal vesicle and calreticulins were distributed evenly throughout the cytoplasm. In mature oocytes, SERCA-ATPases were observed throughout the cytoplasm but were absent from the nuclear region. In contrast, calreticulins were localized mostly in the cortex of the oocyte and were absent from the cytoplasm. However, bright fluoresence stainings were observed in the perimeiotic spindle re-gion of mature oocyte when labeled with antibodies against calreticulin. These results indicate that mouse oocytes undergo distinct rearrangement of the localization of Ca2+-ATPases and calreticulins during meiotic maturation. Thus it can be suggested that redistribution of the Ca2+ stores, as revealed by differential fluorescence stainings, is deeply involved in the regulatory mechanism of mammalian oocyte maturation.