Bacterial infection alters placental ABC transporters expression . These transporters provide fetal protection against circulating xenobiotics and environmental toxins present in maternal blood . We hypothesized that lipopolysaccharide (LPS-bacterial mimic) alters the yolk sac morphology and expression of key ABC transporters in a gestational-age dependent manner . Yolk sac samples from C57BL/6 mice were obtained at gestational ages (GD) 15.5 and GD18.5 , 4 or 24 hours after LPS exposure (150ug/kg; n = 8/group). Samples underwent morphometrical, qPCR and immunohistochemistry analysis . The volumetric proportions of the histological components of the yolk sac did not change in response to LPS . LPS increased Abcg2 expression at GD15.5, after 4 h of treatment (p <0.05). No changes in Abca1, Abcb1a/b, Abcg1, Glut1, Snat1, Il-1ß, Ccl2 and Mif were observed . Il-6 and Cxcl1 were undetectable in the yolk sac throughout pregnancy . Abca1, breast cancer resistance protein (Bcrp, encoded by Abcg2) and P-glycoprotein (P-gp/ Abcb1a/b) were localized in the endodermal (uterine-facing) epithelium and to a lesser extent in the mesothelium (amnion-facing), whereas Abca1 was also localized to the endothelium of the yolk sac blood vessels . LPS increased the labeling area and intensity of Bcrp in the yolk sac's mesothelial cells at GD15.5 (4 h), whereas at GD18.5, the area of Bcrp labeling in the mesothelium (4 and 24 h) was decreased (p <0.05). Bacterial infection has the potential to change yolk sac barrier function by affecting Bcrp and Abcg2 expression in a gestational-age dependent-manner . These changes may alter fetal exposure to xenobiotics and toxic substances present in the maternal circulation and in the uterine cavity.