NOS isoforms (eNOS, iNOS and nNOS) and their mRNA were found in both porcine oocytes and their cumulus cells except for nNOS mRNA, which was not detected in the cumulus cells. NOS isoforms differed in their intracellular localization in the oocyte: while iNOS protein was dispersed in the oocyte cytoplasm, nNOS was localized in the oocyte cytoplasm and in germinal vesicles (GV) and eNOS was present in dots in the cytoplasm, GV and was associated with meiotic spindles. L-NAME inhibitor significantly suppressed metaphase (M)I to MII transition and at the highest concentration also affected GV breakdown (GVBD); in contrast also AG inhibited primarily GVBD.