Objective To study the distribution and expression of interleukin-18(IL-18) in rat uterus during periimplantation and understand possible physiological regulatory roles of endogenous IL-18 in embryo implantation. Methods Immunohistochemical technique and image analysis were used. Results IL-18 protein could be detected in all the rat uteri. The positive cells were mainly distributed in the uterine endometrial or decidual luminal epithelium, the glandular epithelium,and the stroma or decidual cells; a weak expression was also found in the myometrium and some endothelial cells. Compared with the estrus group, the expression of IL-18 in pregnant rats distinctly increased (P<0.05）, and with the developing of pregnancy, the level of IL18 in pregnant rats was gradually enhanced. Moreover, the expression of IL-18 during postimplantation period was markedly higher than that during preimplantation period (P<0.05）.Conclusion IL-18 may play a role in pregnancy and regulate embryo implantation.It m