The p38 MAPK is a member of the mitogen-activated protein kinase (MAPK) family that participates in a signaling cascade in response to cytokines and stress in somatic cells. The present study was designed to investigate the expression and possible function of p38 MAPK in porcine oocytes during maturation. In immunoblots, p38 MAPK was detected in oocytes and cumulus cells. Its activity was determined during oocyte maturation in vitro by the phosphorylation of its substrate, activated transcription factor 2. As ERK1/2, oocyte p38 MAPK became active around germinal vesicle breakdown (GVBD) and maintained activity until metaphase II (MII). Immunofluorescent microscopy showed phosphorylated p38 MAPK accumulated in the nucleus before GVBD and localized in the cytoplasm and around chromosomes from metaphase I (MI) to MII. In cultured cumulus-oocyte complexes, a specific inhibitor of p38 MAPK, SB203580, inhibited phosphorylation of p38 MAPK in cumulus cells and blocked both FSH-induced cumulus expansion and meiotic resumption of oocytes. During spontaneous meiotic resumption of denuded oocytes, SB203580 did not affect GVBD, but it significantly decreased the number of oocytes reaching MII and conversely increased the number of oocytes arrested at MI. These results suggest that p38 MAPK in porcine oocytes becomes active around GVBD, remains active through MI to MII, and has a role in MI-MII transition, and that cumulus p38 MAPK might be involved in FSH-induced meiotic resumption of oocytes.