Since granulocyte elastase has been shown to be involved in the pathogenesis of gastric mucosal lesion formation induced by nonsteroidal antiinflammatory drugs, inhibition of granulocyte elastase release from neutrophils may be useful in the prevention of these lesions. The objective of this study was to determine whether rebamipide inhibits neutrophil activation in vivo and in vitro. Rebamipide and ONO-5046, a specific granulocyte elastase inhibitor, markedly inhibited indomethacin-induced mucosal injury in rats. Gastric myeloperoxidase activity was significantly increased 3 h after indomethacin administration. This increase was significantly inhibited by rebamipide and ONO-5046. Although cimetidine markedly prevented the indomethacin-induced mucosal lesion formation, it did not reduce the gastric myeloperoxidase activity. Rebamipide inhibited granulocyte elastase release from neutrophils in vitro by inhibiting the increase in intracellular Ca2+ level. Cimetidine did not inhibit granulocyte elastase release from neutrophils. Furthermore, the elevation of intracellular Ca2+ level was not inhibited by cimetidine. Therefore, unlike cimetidine, rebamipide may prevent indomethacin-induced mucosal injury by inhibiting neutrophil activation.