Protein kinase A (PKA) play a critical role in maintaining the meiotic arrest. However, the steps downstream of PKA remain largely unknown. In this study, we investigated the regulation of meiotic resumption by PKA/Cdc25B pathway in mouse oocytes. Injection of mRNA coding for Cdc25b-S321A had a more potent maturation-inducing ability than Cdc25b-WT. When co-injected with PKA inhibitor, Cdc25B-WT had similar activities with Cdc25B-S321A. Meanwhile, the phosphorylation of Cdc25B-S321 was detected in germinal vesicle (GV) oocytes by Western blotting with a phospho-Ser321-specific antibody and the band disappeared when oocytes reenter into the meiotic cell cycle. Furthermore, Cdc25B-WT translocated to the nucleus shortly before GV breakdown (GVBD), whereas phosphorylated Cdc25B-S321 expressed exclusively in the cytoplasm and the signal could not be detected in GVBD oocytes. Taken together, these data indicate that Cdc25B-Serine321 is the potential PKA target and Cdc25B subcellular localization determines its function during the process of maintaining GV arrest in mouse oocytes. Developmental Dynamics 237:3777–3786, 2008. © 2008 Wiley-Liss, Inc.