Interleukin-18 (IL-18) belongs to the interleukin-1 family and was identified as an interferon-γ inducing factor. We investigated IL-18 mRNA-expressing cells in the mouse uterus. By RNase protection assay, IL-18 mRNA and α subunit of IL-18 receptor mRNA were detected in the uterus. In the uterus, IL-18 mRNA levels increased during sexual maturation. In situ hybridization analysis demonstrated IL-18 mRNA-expressing cells in the mouse uterus of different ages. At 21 days of age, IL-18 mRNA-expressing cells were detected in the luminal epithelial cells and stromal cells although the IL-18 mRNA signal was weak. At 42 days of age, IL-18 mRNA signal was mainly detected in the stromal cells located near the myometrium, and in some of the luminal and glandular epithelial cells. In the uterus of 63-day-old adult mice, a strong hybridization signal for IL-18 mRNA was detected at estrus, but was weak at diestrus. IL-18 mRNA was mainly detected in the glandular epithelial cells and stromal cells. The effect of estradiol-17β (E₂) on IL-18 mRNA-expressing cells in the uterus was examined in ovariectomized mice. In oil-treated mice IL-18 mRNA signal was localized in luminal epithelial cells and stromal cells, while in E₂-treated mice IL-18 mRNA signal was localized in stromal cells alone. These results suggest that the mouse uterus has an IL-18 system, and IL-18 exerts a physiological role within the uterus in a paracrine manner, and that IL-18 gene expression is regulated by estrogen.