We reported previously that Mater is a maternal effect gene that is required for early embryonic development beyond the two-cell stage in mice. Here we show the expressional profile of Mater and its protein during oogenesis and embryogenesis as well as its subcellular localization in oocytes. Mater mRNA was detectable earliest in oocytes of type 2 follicles, whereas MATER protein appeared earliest in oocytes of type 3a primary follicles. Both mRNA and protein accumulated during oocyte growth. In situ hybridization showed that Mater mRNA appeared progressively less abundant in oocytes beyond type 5a primary follicles. By ribonuclease protection assay, Mater mRNA was abundant in germinal vesicle oocytes, but was undetectable in all stages of preimplantation embryos. In contrast, the protein persisted throughout preimplantation development. Immunogold electron microscopic analysis revealed that MATER was located in oocyte mitochondria and nucleoli, and close to nuclear pores. Taken together, our data indicate that Mater gene transcription and protein translation are active during oogenesis, but appear inactive during early embryogenesis. Thus, Mater and its protein are expressed in a manner typical of maternal effect genes. The presence of MATER protein in mitochondria and nucleoli suggests that it may participate in both cytoplasmic and nuclear events during early development.