The effect of lansoprazole, a proton pump inhibitor (PPI), on indomethacin-induced small intestinal ulceration was examined in rats, particularly in relation to heme oxygenase (HO)-1. The animals were administered indomethacin (10 mg/kg, p.o.) and killed 24 h later. Lansoprazole (30-100 mg/kg, p.o.) and omeprazole (30-100 mg/kg, p.o.) were given 30 min before the administration of indomethacin, while tin-protoporphyrin IX (SnPP: 30 mg/kg, i.v.), an inhibitor of HO-1, was injected 10 min before indomethacin or lansoprazole. Indomethacin produced hemorrhagic lesions in the small intestine, accompanied with an increase of mucosal invasion of enterobacteria, inducible nitric oxide synthase (iNOS) expression, and myeloperoxidase (MPO) activity in the mucosa. Pretreatment with lansoprazole dose- dependently reduced the severity of the indomethacin-induced intestinal lesions, with suppression of the increased MPO activity, while omeprazole had no effect. Pretreatment with SnPP significantly exacerbated these intestinal lesions and almost totally abolished the protective effect of lansoprazole. The up-regulation of iNOS mRNA expression following indomethacin was suppressed by lansoprazole in a SnPP-inhibitable manner, although the enhanced enterobacterial invasion remained unaffected. The amount of HO-1 protein in the intestinal mucosa was significantly increased by lansoprazole but not by omeprazole. Prior administration of carbon monoxide (CO)-releasing molecule-2 (CORM-2; 10 mg/kg, i.p.) significantly reduced the severity of these lesions and the enhancement of mucosal iNOS mRNA expression induced in the small intestine by indomethacin. These results suggest that lansoprazole prevents indomethacin-induced small intestinal ulceration, and this effect is associated with inhibition of iNOS expression, through up-regulation of HO-1/CO production in the mucosa.