Contents P38 mitogen-activated protein kinase (MAPK), which is a member of the canonical MAPK family, is activated in response to various extracellular stresses and plays a role in multiple cellular processes. In this study, we investigated the expression, subcellular localization and functional roles of p38 MAPK during the meiotic maturation of rat oocytes. We found that p38 MAPK phosphorylation (p-p38 MAPK, indicative of p38 MAPK activation) was low at the germinal vesicle (GV) stage, increased 3hr after germinal vesicle breakdown (GVBD) and maintained its maximum at metaphase I (MI) or metaphase II (MII). The p-p38 MAPK mainly accumulated in the GV and had no obvious expression in the nucleus. From GVBD to MII, p-p38 MAPK was distributed in the cytoplasm around either the chromosomes or the spindle. We used SB203580, an inhibitor of p38 MAPK, to investigate the possible functional role of p38 MAPK during rat oocyte meiotic maturation. Treatment of GV stage oocytes with 20M SB203580 blocked p-p38 MAPK activity, and the spindles appeared abnormal. Additionally, the rate of GVBD after 3hr of culture with 20M SB203580 (58.8%) was significantly inhibited compared with the control (82.5%, p<.05), and the polar body extrusion rate after 12hr of culture with SB203580 was also significantly decreased compared with the control (40.1% vs 73.3%, p<.05). Taken together, these data indicate that p38 MAPK may play a vital role in rat oocyte meiotic maturation.