1. We examined the effect of lipopolysaccharide (LPS), a cell wall constituent of Gram negative bacteria, on nuclear factor kappaB (NF-kappaB) activation in the intestine and the roles of endogenous platelet-activating factor (PAF), tumour necrosis factor-alpha (TNF) and neutrophils. We also compared the time course of NF-kappaB activation in response to PAF and LPS. 2. Ileal nuclear extracts from LPS (8 mg kg(-1), IV)-injected rats were assayed for NF-kappaB-DNA-binding activity and identification of the subunits. Some rats were pretreated with WEB2170 (a PAF receptor antagonist), anti-TNF antibody, or anti-neutrophil antiserum. NF-kappaB p65 was localized by immunohistochemistry. An additional group was challenged with PAF (2 microg kg(-1), IV) for comparison. 3. LPS activates intestinal NF-kappaB, both as p50-p50 and p50-p65 dimers within 15 min, and the effect peaks at 2 h. The effect is slower and more sustained than that of PAF, which peaks at 30 min. Activated NF-kappaB was immunolocalized within epithelial and lamina propria cells. LPS effect was reduced by 41, 37 and 44%, respectively, in animals pretreated with WEB2170, anti-TNF antibody, or anti-neutrophil antiserum (P<0.05). 4. LPS activates intestinal NF-kappaB in vivo and neutrophil activation is involved in its action. The LPS effect is mediated by both endogenous PAF and TNF.