The inflammatory and lung maturational effects of intra-amniotic exposure to endotoxin were assessed in fetal lambs. Five hours to 25 days after intra-amniotic injection of endotoxin, preterm lambs were delivered at 119–125 days gestation. Intra-amniotic endotoxin caused an inflammatory cell infiltration in amnion/chorion at 5 h, which persisted for 25 days. At 5–15 h after endotoxin, amnion/chorion cytokine mRNAs increased [12- to 26-fold for interleukin (IL)-1β, IL-6, and IL-8 mRNA and 3-fold for tumor necrosis factor-α mRNA]. At 1–2 days after endotoxin, lung cytokine mRNAs increased 6- to 49-fold. Endotoxin caused modest changes in peripheral white blood cell counts and no significant cytokine mRNA responses in fetal liver, placenta, or jejunum. Lung maturation, as characterized by increased lung volumes and alveolar saturated phosphatidylcholine, occurred at 7 days and persisted for 25 days after endotoxin. We conclude that exposure to a single dose of intra-amniotic endotoxin causes inflammation and increases in cytokine mRNA in amnion/chorion and the fetal lung before lung maturation, consistent with the hypothesis that proinflammatory cytokines signal lung maturation.