We developed a model for the translocation of intraluminal endotoxin in the neonatal animal and used it to examine the capacity of a nonhepatotoxic bile acid, ursodeoxycholic acid (UDCA), to modify endotoxin trans-location and cytokine response. Three-d-old Sprague-Dawley rats were randomized to receive enterally either no drug, lipopolysaccharide (LPS, 1 mg/animal), or UDCA (400 μg/animal) alone, or UDCA followed by LPS 1 h later. One h after LPS administration, the rats were killed and plasma endotoxin and tumor necrosis factor (TNF) were measured. Control animals had low circulating endotoxin (21.2 ± 7.6 endotoxin units) and TNF (0.06 ± 0.02 ng/mL). Enteral administration of LPS 1 h before the rats were killed resulted in significant elevation of endotoxin (249.5 ± 71.3, p = 0.008) and TNF (3.6 ± 1.3, p = 0.019). UDCA alone did not alter endotoxin levels (8.7 ± 2.1). UDCA 1 h before LPS prevented the rise in endotoxin (38.9 ± 11.2 endotoxin units) and TNF (0.2 ± 0.05) significantly. Chenodeoxycholic acid was studied in a similar group of experiments and prevented neither the trans-location of LPS nor the development of increased TNF levels in animals receiving LPS. In conclusion, LPS can cross the intestinal barrier in the normal neonatal rat. UDCA, administered before LPS, can decrease the trans-location of LPS and prevent the cytokine response as measured by TNF levels. We speculate that UDCA, administered prophylactically, might reduce morbidity in clinical conditions leading to gut-derived endotoxemia.