The expression and function of inducible nitric oxide synthase (iNOS) in the stomach is unclear. This study assessed the effects of endotoxin on rat gastric iNOS expression and its role in gastric injury from luminal irritants. In conscious rats, a 5-h treatment with intraperitoneal lipopolysaccharide (LPS; 1-20 mg/kg) dose dependently increased gastric mucosal iNOS immunoreactivity and increased gastric luminal nitrate and nitrite accumulation (Griess reaction). LPS also increased gastric luminal fluid accumulation and reduced macroscopic gastric injury from orogastric acidified ethanol. Aminoguanidine (45 mg/kg) did not prevent LPS-induced gastroprotection or gastric fluid accumulation. NG-nitro-L-arginine methyl ester increased gastric luminal fluid and caused macroscopic gastric injury when given with LPS. Using an anesthetized preparation followed by removal of luminal fluid, LPS reduced gastric mucosal blood flow and exacerbated gastric injury from either acidified ethanol or acidified taurocholate, an effect that was negated by aminoguanidine. These data indicate that in conscious rats, the gastroprotective effect of endotoxin is dependent on constitutive NOS but not iNOS activity. However, the inducible isoform participates in the ability of endotoxin to exacerbate gastric injury from luminal irritants in the anesthetized rat.