Nitric oxide (NO) plays an important role in many pathological processes. The present investigation was undertaken to ascertain the effects of NO at high level on porcine oocyte meiotic development. Cumulusenclosed oocytes (CEOs) and denuded oocytes (DOs) were obtained from the follicles (2-6mm diameter). Exogenous NO was provided with sodium nitroprusside (SNP). Control (0mm), 0.1mm, 1mm or 10mm SNP was introduced to the medium. Meanwhile, 0.1mm, 1mm and 10mm preincubated SNP was also added to the medium respectively. The cumulus expansion, cumulus cells DNA fragmentation, oocyte meiotic maturation and degeneration were determined 44 h after incubation. SNP inhibited cumulus expansion and cumulus cells DNA fragmentation in a dose-dependent manner. Significantly fewer CEOs treated by any level of SNP resumed meiosis than control (p<0.05) and more CEOs exposed to 0.1mm or 1mm SNP were arrested at GV stage. Moreover, SNP increased the percentage of CEOs at metaphase I (MI) stage but decreased the percentage of CEOs at metaphase Ⅱ(MⅡ) stage. 0.1mm or 1mm SNP showed no influence on the proportion of germinal vesicle breakdown of DOs and only 10mm SNP reduced the proportion. SNP accelerated degeneration of both CEOs and DOs, especially at higher concentrations. DOs were more sensitive to SNP toxicity. Preincubated SNP showed no effect on cumulus expansion, cumulus cells DNA fragmentation, oocyte meiotic maturation or degeneration at lower concentration, but higher concentrations had effect, especially on the viability of porcine oocytes. Taken together, high level of NO inhibits porcine oocyte meiotic maturation, especially the transition from MI to MII, by mediating the functions of cumulus cells.