The objective of the present investigation was to examine the spatio-temporal expression of three members of the ETS family of transcription factors, ERM, ER81, and PEA3, in the peri-implantation mouse uterus and in the ovary. These three factors belong to the PEA3 subfamily and are known to mediate diverse functions ranging from neuronal development to tumor progression. As transcription factors, they regulate the expression of a number of genes with various biological functions. Since several genes with known roles in the reproductive processes have been shown to be under the regulation of one of these factors, we sought to investigate the expression of ERM, ER81, and PEA3 in the mouse ovary and uterus. Quantitative RT-PCR analyses showed that ERM, ER81, and PEA3 were all expressed in the peri-implantation mouse uterus, with higher levels of expression on days 4 and 5 of pregnancy. To determine the cell type-specific expression of these factors, we employed in situ hybridization, the results of which revealed that ERM was expressed in both the epithelium and the stroma on days 4 and 5 of pregnancy. Uterine glands showed a high expression of ERM on those days. ERM was also highly expressed in the corpora lutea of the mouse ovary. Both ER81 and PEA3 were expressed at low levels in the stroma on days 4 and 5. On day 8, while ERM and PEA3 were mainly expressed in the embryo and were at low levels in the maternal decidua in a diffused pattern, ER81 was highly expressed in the vascular bed of the mesometrial deciduum. Both ER81 and PEA3 were undetectable in the mouse ovary. Collectively, these data show that ERM is implicated in the early event of implantation as well as in ovarian functions, while ER81 is involved in the establishment of the maternal vasculature for subsequent placental development. PEA3 is apparently an embryonic factor for early embryogenesis.