The concept that gastroprotection by agents such as mild irritants, antacids, or sucralfate is prostaglandin (PG)-mediated has been challenged recently. These agents do not reproducibly stimulate prostaglandin formation, and indomethacin does not effectively attenuate their protective potency. Rebamipide is a novel antiulcer compound. This study was designed to clarify whether eicosanoids contribute to the gastroprotective activity of the drug. In the rat stomach, rebamipide (100 and 500 mg/kg, intraperitoneally) slightly increased release of PGE2, 6-keto-PGF1 alpha, thromboxane B2, and the metabolite 15-keto-13,14-dihydro-PGE2 from mucosal fragments incubated ex vivo and significantly enhanced secretion of these products into the lumen, resulting in gastric juice eicosanoid levels exceeding those in controls several-fold. Mucosal formation of leukotriene (LT) C4 was not affected by rebamipide. Rebamipide caused substantial protection against gastric damage produced by ethanol, which was antagonized by pretreatment with indomethacin (0.1-5 mg/kg, subcutaneously). The dose-response relationship of indomethacin for inhibition of prostaglandin formation and rebamipide-induced protection correlated well and 5 mg/kg indomethacin completely prevented the protective effect of rebamipide. The results indicate that: (1) in contrast to most other protective agents, protection by rebamipide involves the endogenous prostaglandin system; (2) the increase in prostaglandin formation results from stimulation of biosynthesis, and not inhibition of degradation; (3) gastroprotection by rebamipide occurs despite increased thromboxane formation and is not associated with reduced generation of LTC4; and (4) determinations of gastric juice eicosanoids seem to be particularly useful to evaluate effects of agents increasing formation of cyclooxygenase products in the stomach.