Sperm nuclear decondensing activity in mammalian oocytes is dependent upon the maturational state of the oocyte. It is maximal in mature, metaphase II oocytes and minimal or absent in immature germinal vesicle (GV) and fertilized pronuclear oocytes. Previous studies suggested that this difference may be due to the relative ability of an oocyte to reduce the protamine disulfide bonds in the sperm nucleus. The results of the present study show that mature hamster oocytes contain significantly more glutathione (GSH), about 8 mM, and hence more disulfide reducing power, as compared with GV (4 mM) or pronuclear (6 mM) oocytes. Furthermore, the acquisition of sperm nuclear decondensing activity by maturing oocytes can be prevented or delayed by blocking GSH synthesis with L-buthionine-S,R-sulfoximine during the early stages of oocyte maturation. This is the first evidence that modulation of GSH levels during oocyte maturation and fertilization may be a mechanism by which sperm nuclear decondensing activity is regulated.