The migration, adhesion, and proliferation of conceptuses during pregnancy are tightly controlled processes that are mediated by various factors including cytokines, growth factors, and hormones. Among many factors, chemokines play key roles in lymphocyte trafficking, cellular proliferation, vascularization, and embryogenesis in many mammalian species. Especially, it has been shown that C-X-C chemokine ligand 12 (CXCL12) plays an important role in early pregnancy by promoting trophoblast invasion, proliferation, and differentiation through its receptor, C-X-C chemokine receptor 4 (CXCR4) in humans. However, expression and function of CXCL12 in the uterine endometrium during pregnancy have not been well studied in pigs. Thus, we determined expression of CXCL12 and its receptor, CXCR4, in the uterine endometrium during the estrous cycle and pregnancy in pigs. We obtained endometrial tissues from gilts on day (D) 12 and D15 of the estrous cycle and D12, D15, D30, D60, D90, and D114 of pregnancy, conceptus tissues from D12 and D15 of pregnancy, and chorioallantoic tissues from D30, D60, D90, and D114 of pregnancy. Real-time RT-PCR analysis showed that levels of CXCL12 and CXCR4 mRNAs changed in the uterine endometrium during pregnancy. Levels of CXCL12 and CXCR4 mRNAs on D15 of pregnancy were higher than those on D15 of the estrous cycle. After D15 of pregnancy levels of CXCL12 and CXCR4 mRNAs gradually decreased toward term of pregnancy, and CXCL12 and CXCR4 were expressed in the chorioallantoic tissues during the mid- to late pregnancy. CXCL12 and CXCR4 mRNAs were expressed in chorioallantoic tissues during mid- to late pregnancy, and RT-PCR analysis showed that CXCL12 and CXCR4 mRNAs were detectable in conceptus on D12 and D15 of pregnancy. Immunohistochemistry showed that CXCL12 proteins were localized to endometrial luminal and glandular epithelial cells during the estrous cycle and pregnancy, and to chorionic epithelial cells during mid- to late pregnancy. Abundance of CXCL12 mRNAs, but not CXCR4, in the uterine endometrium was increased by the treatment of IFNG. These results showed that CXCL12 and CXCR4 were expressed in the uterine endometrium, conceptus, and chorioallantoic tissues and IFNG increased endometrial CXCL12 expression in pigs, suggesting that CXCL12 and its receptor may play a key role in regulation of the establishment and maintenance of pregnancy by affecting the conceptus development in pigs. [supported by the Next Generation BioGreen 21 Program (#PJ01110301), Rural Development Administration]