Galectin-1 is a member of beta-galactoside-binding lectins expressed in a variety of mammalian tissues. We report here that galectin-1 mRNA is abundantly expressed in the mouse reproductive organs such as the uterus and ovary. Uterine expression of galectin-1 mRNA is specifically regulated in the embryonic implantation process. Its expression increased at a high level on the fifth day post coitum (dpc 5) when embryos hatched into the endometrial epithelial cells. In the absence of embryos, however, galectin-1 expression in the mouse uterus decreased on dpc 5. In the delayed implantation mice, galectin-1 mRNA levels was augmented by the termination of the delay of implantation. Ovarian steroids progesterone and estrogen differentially regulated galectin-1 mRNA level in uterine tissues. Treatment with RU486, a progesterone receptor antagonist, blocked progesterone-induced galectin-1 mRNA level in uterine tissues of ovariectomized mouse. ICI182780, a pure estrogen receptor antagonist, clearly blocked the estrogen effect. Taken together, galectin-1 gene expression in the uterine tissues was regulated by ovarian steroids and this regulation correlated with the implantation process.