Endotoxin [lipopolysaccharide (LPS)] from Gram-negative bacteria is found in amniotic fluid in intrauterine infections that associate with the risk for spontaneous premature birth, bronchopulmonary dysplasia (BPD), and respiratory distress syndrome. Toll-like receptor 4 (TLR4) is the signaling receptor for LPS. The aim was to investigate the primary inflammatory response in mice shortly after administration of LPS to the dam (14 and 17 d of pregnancy), to the newborn, or into the amniotic fluid. The expression levels of TLR4, IL-1, tumor necrosis factor-alpha, IL-6, IL-10, macrophage inflammatory protein-2, and IL-1 receptor 1 were studied with ribonuclease protection assay. In addition, TLR4 protein was analyzed with Western blotting. The fetal membranes expressed TLR4 mRNA and protein and showed an acute cytokine response to LPS when LPS was administrated into the amniotic fluid. There was distinct ontogeny in the responsiveness of fetal lung to LPS: on fetal day 14 (term 20 d), both the expression of TLR4 and the acute cytokine response were undetectable 5 h after LPS; they became detectable by fetal day 17. TLR4 and the cytokine response further increased after birth. In maternal lung, the TLR4 expression was strongest and up-regulated in parallel with the induction of the cytokines. We propose that TLR4 controls the magnitude of the LPS-induced cytokine response during the perinatal period.