The present experiments aimed to investigate the metabolism of calcium during oocyte maturation in rat. The concentration of free calcium and calmodulin in oocytes was measured respectively by using of fluo-3/AM and FITC with microscope fluorescence spectrometer. The ultrastructural localization of calcium precipitates in oocytes was observed with the transmission electron microscope. Cumulus-free immature oocytes(GV-oocyte) were cultured in vitro through 15 hours. The free calcium concentration in GV oocyte was $55.9{pm}3.5nM$. In calcium-containing medium, the free calcium concentration was increased in germinal vesicle breakdown(GVBD) oocyte($64.2{pm}7.3nM$). In normal medium after calcium chelator treatment ($10{mu}M$ BAPTA/AM), the free calcium contents were slightly lower than those in control group. In calcium-free medium, the free calcium content was drastically increased in GVBD($72.7{pm}3.4nM$) and metaphase I - anaphase I ($88.0{pm}3.4nM$) oocyte. In maturation rate of oocytes, GVBD rate was high in control group($82.9{pm}6.55%$) and calcium chelator treatment group($91.2{pm}4.4%$), but in calcium-free medium group, it was low and then the oocyte was degenerated without polar body formation. Relative content of calmodulin in oocyte was significantly(P<0.001) increased in metaphase I - anaphase I than in GV and GVBD oocyte. The calcium precipitates were observed in mitochondria and cytoplasm of GV oocyte but that were not observed in mitochondria of GVBD and metaphase I - anaphase I oocyte. And then the calcium precipitates reappeared in mitochondria of metaphase II oocyte. The above results indicate that changes in free calcium and calmodulin concentration of oocyte occur according to the maturational stages and the extracellular calcium is required during oocyte maturation. Also change of calcium localization in oocyte occurs according to the maturational stages.