In an effort to better understand oocyte function, we utilized two‐dimensional (2D) electrophoresis and mass spectrometry to identify proteins that are differentially expressed during murine oocyte maturation. Proteins from 500 germinal vesicle (GV) and metaphase II‐(MII) arrested oocytes were extracted, resolved on 2D electrophoretic gels, and stained with silver. Analysis of the gels indicated that 12 proteins appeared to be differentially expressed between the GV and MII stage. These proteins were then cored from the 2D gels and identified by mass spectrometry as: transforming acidic coiled‐coil protein 3 (TACC3), heat shock protein 105 (HSP105), programmed cell death six‐interacting protein (PDCD6IP), stress‐inducible phosphoprotein (STI1), importin α2, adenylsuccinate synthase (ADDS), nudix, spindlin, lipocalin, lysozyme, translationally controlled tumor protein (TCTP), and nucleoplasmin 2 (NPM2). Interestingly, PDCD6IP, importin α2, spindlin, and NPM2 appear slightly larger in mass and more acidic on the MII oocyte gel compared to the GV oocyte gel, suggesting that they may be post‐translationally modified during oocyte maturation. Given NPM2 is an oocyte‐restricted protein, we chose to further investigate its properties during oocyte maturation and preimplantation development. Real‐Time RT‐PCR showed that NPM2 mRNA levels rapidly decline at fertilization. Indirect immunofluorescence analysis showed that, with the exception of cortical localization in MII‐arrested oocytes, NPM2 is localized to the nucleus of both GV stage oocytes and all stages of preimplantation embryos. We then performed one‐dimensional (1D) western blot analysis of mouse oocytes and preimplantation embryos and found that, as implicated by the 2D gel comparison, NPM2 undergoes a phosphatase‐sensitive electrophoretic mobility shift during the GV to MII transition. The slower migrating NPM2 form is also present in pronuclear embryos but by the two‐cell stage, the majority of NPM2 exists as the faster migrating form, which persists to the blastocyst stage. Mol. Reprod. Dev. 74: 608–616, 2007. © 2006 Wiley‐Liss, Inc.