Oligopeptide hormones are involved in cell–cell interaction during embryonal implantation and neuropeptide Y (NPY) is expressed in the human placenta and decidual cells in the third trimester of pregnancy. However, there is no report regarding the intrauterine localisation and the functions of NPY during the peri-implantation period. In the present study, the spatiotemporal changes in NPY expression in the murine uterus during the peri-implantation period were investigated using reverse transcription–polymerase chain reaction (RT-PCR), quantitative RT-PCR and immunohistochemical techniques, as were the effects of sex steroids on NPY mRNA expression in primary cultured murine uterine epithelial cells. Neuropeptide Y mRNA was increased in the pregnant murine uterus, as well as in the pseudopregnant murine uterus, during the peri-implantation period. Immunohistochemical analysis revealed increases in NPY expression in luminal and glandular epithelial cells and decidualised stromal cells. Neuropeptide Y mRNA expression was strongly induced in cultured epithelial cells in response to sex steroids. The data suggest that NPY is involved in cell–cell interactions during embryonic implantation.