The roles of nitric oxide (NO) and serotonin (5-HT) in the development of gastric mucosal lesions induced by compound 48/80 (48/80) were investigated in rats. Repeated i.p. administration of 48/80 (1 mg/kg) produced damage in the stomach with severe oedema in the submucosa. The lesions induced by 48/80 were prevented by FPL-52694 (a mast cell stabilizer) and methysergide but not tripelennamine. The lesions were also inhibited by simultaneous administration of NG-monomethyl-L-arginine (L-NMMA), and this effect was mimicked by inducible NO synthase (iNOS) inhibitors, such as aminoguanidine or dexamethasone and significantly antagonized by coadministration of L-arginine. The mucosal myeloperoxidase activity, thiobarbituric acid reactants and vascular permeability in the stomach were all increased after 48/80 treatment and the changes were also attenuated by cotreatment with L-NMMA. Repeated s.c. treatment with 5-HT (20 mg/kg) provoked similar gastric lesions, which were also prevented by methysergide and iNOS inhibitors, as well as antioxidative drugs, such as allopurinol (a xanthine oxidase inhibitor) and hydroxyurea (a neutrophil-reducing agent). The Ca2+-independent NO synthase (NOS) activity was increased in the gastric mucosa after administration of 48/80 or 5-HT and this change was inhibited by dexamethasone. These results suggest that: (i) the repeated administration of 48/80 induced inflammatory gastric lesions in the rat stomach, mediated by endogenous 5-HT; (ii) NO/iNOS is involved in the pathogenic mechanism of 48/80-induced gastric lesions, in addition to oxyradical formation; and (iii) the deleterious role of NO in this lesion model can be accounted for by a cytotoxic action of peroxynitrite that is formed in the presence of superoxide radicals.