Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1 α and PP1 γ . In addition, the transcript for PP2A was also present. At the protein level only PP1 α and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1 γ spliced variant proteins, PP1 γ1 and PP1 γ2 , was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1 α and PP2A, but not PP1 γ , in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1 α and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1 α , although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1 α and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1 α and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1 α into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1 α and PP2A in mouse oocytes. The differential localization of PP1 α and PP2A, in addition to the relocation of PP1 α during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.