To elucidate the role of cyclooxygenase (COX)-2 in ulcer healing, we compared the effects of NS-398 (COX-2-selective inhibitor) and indomethacin (nonselective COX inhibitor) on the healing of acetic acid-induced gastric ulcers in rats. Prostaglandin E₂(PGE₂) production was elevated in ulcerated tissue, but remained unaffected in intact tissue. COX-2 mRNA was only detected in the ulcerated tissue, in which the COX-2 protein was found in fibroblasts, macrophages/monocytes and granulocytes. In contrast, COX-1 mRNA expression was not affected by ulceration. In an in vitro study, the increased PGE₂ production was inhibited by NS-398; this had no effect on PGE₂ production in the intact tissue. When NS-398 and indomethacin were administered to rats, 3 and 6 mg/kg NS-398 only reduced PGE₂ production in the ulcerated tissue, but 10 mg/kg NS-398 and 0.5 to 2 mg/kg indomethacin inhibited the production in both the ulcerated and intact tissues. The healing of gastric ulcers was significantly impaired by 3 to 10 mg/kg NS-398 and 1 and 2 mg/kg indomethacin. The delay in ulcer healing was associated with the inhibition of PGE₂production in the ulcerated tissue. As observed upon histological analysis, regeneration of the mucosa, maturation of the ulcer base and angiogenesis in the base were significantly prevented by 6 mg/kg NS-398 and 2 mg/kg indomethacin, although the inhibitory effect of NS-398 was weaker than that of indomethacin. These results clearly indicate that COX-2 plays an important role in the healing of gastric ulcers in rats.