1. Helicobacter pylori (Hp) infection, which involves the gastric antrum and duodenal mucosa, may be involved in peptic ulceration by stimulating the local release of cytoxic or pro-inflammatory factors. 2. Nitric oxide (NO) is known to be cytotoxic at high concentration. The aim of the present study was therefore to investigate the ability of a water soluble extract of Hp to induce NO synthase in duodenal mucosa and epithelial cells following its administration in vivo in rats and determine its association with cell damage. 3. Administration of Hp water extract (4 ml kg(-1)) led to the expression of the calcium-independent inducible nitric oxide synthase (iNOS) after 4 h in the duodenum, determined as [14C]-arginine conversion to citrulline. 4. This iNOS activity was not reduced by pretreatment with anti-neutrophil serum (0.4 ml kg(-1), i.p., 3 h before challenge). However, dexamethasone pretreatment (1 mg kg(-1), i.v., 2 h before the extract), or administration of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 5 mg kg(-1), i.v., 2.5 h after the extract) reduced this activity. 5. Furthermore, iNOS was expressed in duodenal isolated epithelial cells 4 h after the i.v. challenge with the extract, at a time when the cellular viability was also reduced, as assessed by trypan blue exclusion. 6. Dexamethasone pretreatment, administration of L-NAME, or pretreatment with polymyxin B (1 mg kg(-1), i.v.) which binds endotoxin, reduced both the iNOS activity and epithelial cell damage. 7. The induction of NO synthase by the Hp extract thus results in duodenal epithelial cell injury and such actions could play a role in pathogenesis of peptic ulcer disease.