Cysteamine induces perforating duodenal ulcers in rats within 24-48 hr. This reducing aminothiol generates hydrogen peroxide in the presence of transition metals (e.g., ferric iron), producing oxidative stress, which may contribute to organ-specific tissue damage. Since most intestinal iron absorption takes place in the proximal duodenum, we hypothesized that cysteamine may disrupt regulation of mucosal iron transport, and iron may facilitate cysteamine-induced duodenal ulceration. We show here that cysteamine-induced ulceration was aggravated by pretreatment of rats with Fe³⁺ or Fe²⁺ compounds, which elevated the iron concentration in duodenal mucosa. In contrast, feeding rats an iron-deficient diet was associated with a 4.6-fold decrease in ulcer formation, accompanied by a 34% decrease (p<0.05) in the duodenal mucosal iron concentration. Administration of deferoxamine inhibited ulceration by 65%. We also observed that the anti-ulcer effect of H-2 receptor antagonist cimetidine included a 35% decrease in iron concentration in the duodenal mucosa. Cysteamine-induced duodenal ulcers were also decreased in iron-deficient Belgrade rats (p<0.05). In normal rats, cysteamine administration increased the iron concentration in the proximal duodenal mucosa by 33% in the pre-ulcerogenic stage but at the same time decreased serum iron (p<0.05). Cysteamine also enhanced activation of mucosal iron regulatory protein 1 (IRP1) and increased the expression of divalent metal transporter-1 (DMT1) mRNA and protein. Transferrin receptor 1 (TfR1) protein expression was also increased, although mucosal ferroportin and ferritin remained almost unchanged. These results indicate an expansion of the intracellular labile iron pool in the duodenal mucosa, increasing its susceptibility to oxidative stress, and suggest a role for iron in the pathogenesis of organ specific tissue injury such as duodenal ulcers.