We investigated the roles of endogenous leukotrienes (LTs) and prostaglandins (PGs) in the healing of gastric ulcers induced by acetic acid in rats. The mucosal levels of LTB4 and sulfidopeptide LT at the ulcer edge had increased by one day after the induction of ulcers. AA-861, a selective inhibitor of 5-lipoxygenase, did not affect the ulcer healing. Indomethacin delayed the healing. Cimetidine did not affect this delay, but ornoprostil, a PGE1, derivative, prevented it. These results suggest that endogenous LTs are not related to the healing of gastric ulcers and that a deficiency in endogenous PGs may be involved in the persistence of gastric ulcers.