The leukotrienes, platelet activating factor and intracellular calcium have been implicated in the development of gastro-intestinal lesions induced by non-steroidal anti-inflammatory drugs (NSAIDs) but the relative significance of these inflammatory mediators in lesion formation has not been established in sensitive and specific models of gastro-intestinal ulceration. In the present study the effects of drugs affecting 5-lipoxygenase activity, the actions of platelet activating factor and intracellular calcium on the development of gastric and intestinal ulceration induced by NSAIDs were investigated in highly sensitive models of ulcerogenicity induced by treatment with either the cholinomimetic, acetyl-beta-methyl choline chloride, in mice (gastric mucosal lesions) or adjuvant-induced polyarthritis in rats (gastric and intestinal mucosal lesions) as well as in normal mice (intestinal mucosal lesions). The 5-lipoxygenase inhibitors, such as MK-886 (3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2-+ ++dimethylpropanoic acid), given at doses shown to reduce the indomethacin-induced increase in mucosal leukotriene B4 concentrations were found to partially prevent the development of gastric and intestinal lesion induced by indomethacin and gastric lesions from aspirin, but the same doses of MK-886 did not affect gastric lesions from diclofenac. Pretreatment with these inhibitors at both 3-5 h and 0-0.25 h was required to achieve protection against gastric mucosal damage from indomethacin. Immediate prior administration of platelet activating factor antagonists (e.g. WEB-2086) with the 5-lipoxygenase inhibitors did not affect gastric or intestinal lesions induced by indomethacin. The calcium antagonist, verapamil, was slightly protective against gastric and intestinal lesions induced by indomethacin. Gastric lesions were further prevented by combinations of a single dose of verapamil with a platelet activating factor antagonist but not combined with a 5-lipoxygenase inhibitor; other combinations of verapamil with lipoxygenase inhibitors or platelet-activating factor antagonists being without inhibitory effects on gastric or intestinal lesions compared with the drugs alone. These results show that 5-lipoxygenase products and intracellular calcium play a major role in acute gastric and intestinal damage by the NSAIDs, but platelet-activating factor has little or no appreciable involvement.