To investigate the ability to decondense sperm head penetrated into cytoplasm of the oocytes and the relationship between this ability and the level of glutatione (GSH) in mouse oocyte at various maturing stages. The fertilizability of oocytes at various stages of maturation the decondensation of sperm nucleus and the formation of male pronucleus, were observed and the levels of GSH were measured in oocyte at same stages. Besides, the relation between fertilizability and level of GSH in oocyte cytoplasm treated with L-buthionine-S, R-sulfoxmine (L-BSO), the inbitor of biosynthesis of GSH, was determined. The decondensation of sperm head was not found in GV stage and L-BSO treated oocytes. In maturing oocytes (GVBD, MI), the decondensation was found, but the formation of male pronucleus was not. The levels of GSH in oocyte cytoplasm were measured; 2.2 pmol per oocyte in the ovulated and the matured in vitro each, 1.0 pmol in GV intact oocyte, 1.3 pmol in GVBD, and 1.5 pmol in MI phase oocyte. In L-BSO treated oocytes the levels of CSH were measured 0.08~o.09 pmol per oocyte, slightly lower than GV stage oocyte. In conclusion, GSH in oocyte is supposed to be synthesized and storaged in cytoplasm during maturation. The failure of decondensation in the cytoplasm of GV stage and L-BSO treated is suggested that GSH is an essential factor in decondensing the sperm head and that the a certain level of GSH, more than in GV oocyte cytoplasm, is required in decondensation.