The present experiment used cultured mouse cumulus cell-enclosed oocytes (CEOs) and denuded oocytes (DOs) to study the function of nitric oxide (NO) in mouse oocyte meiotic maturation. Either positive or negative actions of NO on meiotic maturation has been observed when CEOs or DOs were cultured for 24h in a medium containing 4mM hypoxanthine (HX) to maintain meiotic arrest, or in maturation medium (without HX) supplemented with different doses of sodium nitroprusside (SNP, a NO donor), Nomega-nitro-L-arginine methyl ester (L-NAME) or Nw-nitro-L-arginine (L-NNA) (two inhibitors of NO synthase, NOS), and Larginine (the only substrate of NOS). Both NOS inhibitors suppressed the formation of first polar body (PB1) of the oocytes in CEOs in a dose dependent manner, but no effect on germinal vesicle break down (GVBD) was observed. An optimal inhibitory effect was observed with either 10-3M L-NAME (PB/ 0.01) or 10-3M L-NNA (PB/0.01) and the inhibition could be reversed by the addition of SNP (10-5M). The above mentioned optimal concentration of L-NAME or L-NNA on CEOs exhibited no effect on oocyte meiotic maturation of DOs. Treatments of low concentrations of SNP (10-7, 10-6, 10-5M) stimulated significantly the oocyte meiotic maturation of CEOs which were inhibited with HX, but had no effect on DOs in the same culture medium. While, the treatment with high concentrations of SNP (0.1/4mM) during the CEOs cultured in maturation medium resulted in a lower percentage of oocytes at PB1 stage and a higher percentage of atypical oocytes in a dose dependent manner compared with control. A dose of SNP at 1mM exhibited significant inhibitory effect on the formation of PB1, but without effect on the number of atypical oocytes compared with control, while, this SNP dosage not only inhibited the oocyte PB1 formation but also increased the percentage of dead oocytes in DOs. Although oocytes of all groups underwent GVBD at the end of the culture in the spontaneous maturation medium, the results of the kinetics showed that the treatment of the optimal concentration of SNP (1mM) could significantly delay GVBD during the first 5h culture period. The concomitant addition of L-NAME with SNP did not reverse the inhibitory effect of SNP on CEOs. Similarly, neither pre-incubation nor illumination by ultraviolet ray could balance the inhibitory effect of SNP. Finally, when added alone at a concentration of 4mM, L-arg caused extensive death of both CEOs and DOs. While, administration of 4mM L-arg and 1mM L-NAME to both CEOs and DOs simultaneously resulted in markedly reduced CEOs death percentage as compared with L-arg treatment alone, but not in DOs. These data support the idea that NO could act with a dual action (stimulation or inhibition) in mouse meiotic maturation depending on its concentration.