The effect of sodium nitroprusside (SNP), a donor of nitric oxide, on meiotic maturation of mouse oocytes was studied by injecting Nw-nitro-L-arginine methyl ester (L-NAME) intra-peritoneal (ip), a nitric oxide synthase inhibitor, or culturing oocytes in the medium supplemented with L-NAME or hypoxanthine (HX) to arrest the spontaneous oocyte maturation in vitro. The results showed that the inhibitory effect of L-NAME by injecting 10mg/kg ip on extrusion of the first polar body only could be reversed by injecting 2.5mg/kg SNP with L-NAME simultaneously (P<0.05). Half an hour later ten mice died when given 10mg/ kg SNP ip. The treatment of some concentrations of SNP (10-7, 10-6, 10-5mol/L) significantly stimulated meiotic maturation to metaphase Ⅱ stages in cumulus enclosed ocytes in the presence of HX. However, other concentrations of SNP (10-8, 10-4, 10-3mol/L) had no effect on HX-arrested oocyte meiotic maturation. The optimal concentration of SNP on CEOs had no effect on DOs. The dose of 10-3 mol/LL-NAME demonstrated a significant suppression in formation of PB1, but not in GVBD. This inhibition was reversed by the addition of SNP. These results indicated that the siological levels of NO produced by cumulus cells could stimulate meiotic maturation of mouse oocytes both in vivo and in vitro.