Previous studies have shown that prematurity, enteral feeding, bacterial colonization, and intestinal ischemia are important factors in the development of neonatal necrotizing enterocolitis (NEC). It has been theorized that increased mucosal permeability leading to endotoxinemia may be a key inciting event in the etiology of NEC. In this study, we investigated the effects of enteral tube feeding, asphyxia, and cold stress on the permeability of the neonatal rat intestinal mucosa and the development of endotoxinemia. Neonatal Sprague-Dawley rats (n=11) were delivered by c-section, and warmed in a neonatal incubator. At 30 min all pups were enterally tube fed 750mg fluorescein isothiocyanate (FITC)-dextran (mol. wt. 73,100)/kg body weight(previous studies have shown that the 73,000 mw compound differentiated newborn rat permeability more consistently than 11,000 or 4,000 mw dextran). Group A (n=4) pups were neither fed nor stressed. Group B (n=3) and group C(n=4) were formula fed (0.2 ml/rat) via orogastric tube at 60 min and again at 195 min. At 105 min and again at 180 min Group C pups were placed in a hypoxia chamber for two min followed by a cold chamber (4°C) for 12 min. At 315 min, animals were euthanized and plasma collected for permeability measurement and endotoxin assay. Mucosal permeability was estimated by measuring plasma FITC-dextran concentrations using spectrophotofluorometric methods. Plasma endotoxin concentration was measured using the Limulus Amebocyte Lysate Assay. Mean FITC-Dextran results reported in Flourescence Transmittal Units (FTU) were as follows: Group A=0.08 FTU, B=0.34 FTU, C=0.44 FTU. In addition, we found that formula feeding increased mean plasma endotoxin concentrations as follows: A=0.60 EU/ml, B=7.16 EU/ml, C=3.28 EU/ml. These data suggest that enteral feeding alone increases the permeability of the neonatal intestinal mucosa and allows for the development of endotoxinemia. Since asphyxial stress as used in this protocol did not alter the intestinal mucosal permeability, alternative mechanisms may explain the importance of asphyxia on the development of NEC in this model. Supported in part by NIH grant HD00999 and a grant from The March of Dimes.