Interleukin-18 (IL-18) is a proinflammatory cytokine expressed in female reproductive organs in humans, rats and mice. The physiological roles of uterine IL-18 and the regulatory mechanisms of IL-18 gene expression are unclear. The present study aimed to clarify the effects of estradiol-17β (E2) and progesterone (P4) on IL-18 mRNA expression in the mouse uterus. Distribution and expression levels of IL-18 mRNA were studied using an RNase protection assay. Expression of IL-18 mRNA was observed in all organs studied, including testes, ovaries and uteri. The uterine IL-18 mRNA level of estrous mice was higher than that of diestrous mice. E2 treatment (1, 5, 25 or 250 ng/mouse) decreased uterine IL-18 mRNA levels in ovariectomized mice dose-dependently. E2 treatment acutely decreased IL-18 mRNA levels 3 h after injection, but these levels returned to the initial level after 48 h. P4 treatment (1 mg/mouse) decreased uterine IL-18 mRNA levels after 12 h, but levels returned to the initial level after 48 h. Both uterine IL-18 and IL-18Rα mRNAs were detected in cultured endometrial epithelial and stromal cells. These results suggest that uterine IL-18 expression is reduced by sex steroid hormones and that IL-18 acts on endometrial cells in a paracrine or autocrine manner.