Tumor necrosis factor alpha (TNF-alpha) has been suggested to play a critical role in indomethacin-induced gastric mucosal damage, so we evaluated its mucosal level and its relationship with prostaglandin E2 and neutrophils in indomethacin-induced gastric mucosal injury in rats. Indomethacin caused a time- and dose-dependent increase in gastric mucosal erosion, which was accompanied by a reduction in prostaglandin E2 followed by an increase in TNF-alpha level and neutrophil infiltration in the gastric mucosa. Pretreatment with exogenous prostaglandin E2 totally abolished indomethacin-induced gastric mucosal injury and the TNF-alpha increase. Depletion of neutrophils by methotrexate or reduction of TNF-alpha concentration by pentoxifylline markedly reduced indomethacin-induced mucosal damage. Pentoxifylline but not methotrexate prevented the increase in mucosal TNF-alpha level induced by indomethacin. It is suggested that depletion of prostaglandin E2 followed by an increase of TNF-alpha production and neutrophil infiltration in the gastric mucosa are important sequential processes in indomethacin-induced ulceration. Prevention of one of these processes would inhibit ulcer formation.