Embryo implantation involves a complex regulatory network of steroid hormones, inflammatory cytokines, and immune cells. Lipoxin A(4) (LXA(4)), a biologically active eicosanoid with specific anti-inflammatory and pro-resolving properties, was recently found to be a novel modulator of estrogen receptor alpha (ER alpha). In this study, we investigated the potential role of LXA(4) in implantation. We found that LXA(4) blocked embryo implantation in mice and significantly reduced the expression of inflammatory mediators associated with uterine receptivity and embryo implantation, including corticotropin-releasing factor (CRF), cyclooxygenase 2-derived prostaglandin I-2 and prostaglandin E-2, leukemia inhibitory factor, and interleukin 6, but this effect was independent of LXA4 receptor. Subsequent investigation revealed enhanced ER alpha activity in the uteri of LXA(4)-treated mice during the peri-implantation period. ER alpha and phosphorylated ERa were significantly increased following LXA(4) treatment. Finally, it was demonstrated that the inhibitory effect of LXA(4) on embryo implantation was mediated through ER alpha. In the presence of the ER alpha antagonist ICI 182 780, LXA(4) failed to block embryo implantation. LXA(4) also failed to inhibit CRF expression. These results suggested that LXA(4) blocks embryo implantation by controlling ERa activity, and this effect appeared to be related to the suppression of the inflammatory microenvironment necessary for implantation.