In this study, the effects of Centella asiatica water extract (CE) and its active constituent, asiaticoside (AC), on the expression and activity of inducible nitric oxide synthase (iNOS) during gastric ulcer healing in rats were investigated. CE was prepared from Centella asiatica dry plant and the concentration of AC in CE was quantitatively determined with the use of high performance liquid chromatography analysis. Different concentrations of CE (0.10 g/kg and 0.25 g/kg) and AC (5 mg/kg and 10 mg/kg) were orally administered to rats with acetic acid-induced gastric ulcers. They were found to reduce the size of the ulcers at days 1, 3 and 7 after ulcer induction in a dose-dependent manner, with a concomitant attenuation of iNOS activity and protein expression at the ulcer tissues. The levels of nitrite and nitrate (NO(X)-), the stable end-products of nitric oxide (NO), in the gastric ulcer tissues were also decreased. N-[3-(aminomethyl)benzyl]acetamidine (1400W), a highly selective inhibitor of iNOS, was found to produce similar but more potent inhibition on iNOS activity at a dose of 0.1 mg/kg. These findings indicate that CE and AC have an anti-inflammatory property that is brought about by inhibition of NO synthesis and thus facilitates ulcer healing.