Platelet-activating factor (PAF), an endogenous pro-inflammatory phospholipid, when injected intravascularly to rats and mice, causes shock, acute bowel injury and a rapid activation of nuclear factor-B (NF-B) p50-p50 with up-regulation of the chemokine CXCL2 in the intestine. In this study, we investigate the mechanism of NF-B activation and the role of the NF-B p50 subunit in PAF-induced shock and acute bowel injury. NF-B p50-deficient mice and wild-type mice were anesthetized, tracheotomized and their carotid artery was cannulated for blood pressure monitoring, blood sampling and PAF administration. For determination of bowel injury, shock and survival, PAF (2.2µg/kg, intra-arterially (i.a.)) was injected. Two hours later, animals were euthanized and their small intestine was removed for histological examination. For biochemical studies, PAF (1.5 µg/kg, i.a.) was administered and the small intestine removed after 15 to 60 min. We found that PAF induced an increase in p105 processing within 30 min but there were no changes in the levels of the NF-B inhibitory proteins IB and . NF-B p50-deficient mice were protected against PAF induced mortality, shock, intestinal hypoperfusion and injury compared to wild-type animals. We also found that p50-deficient mice had decreased gene expression of CXCL2 and TNF and a decrease in CXCL2 protein production, compared to wild-type mice. Our study suggests that PAF increases the processing of NF-B p105 into p50, with up-regulation of pro-inflammatory cytokines, which, leads to PAF-induced systemic inflammatory response and acute bowel injury.