In this investigation three bioactive compounds, responsible for the gastroprotective property of Amphipterygium adstringens, were purified from an active dichloromethane fraction. These compounds were 3alpha-hydroxymasticadienonic acid, beta-sitosterol and 3- epi-oleanolic acid. The latter was the most active compound (88.8 % of gastroprotection) followed by 3alpha-hydroxymasticadienoic acid and beta-sitosterol (69.8 and 42.5 % of gastroprotection, respectively). Carbenoxolone was used as positive control and it showed 88.4 % of gastroprotection. Masticadienonic acid was also isolated from the active fraction, but it was unable to inhibit the ethanol-induced gastric lesions. The gastroprotection of the methanol extract was completely inhibited by the pretreatment with l-NAME and attenuated by pretreatment with indomethacin and N-ethylmaleimide. These results suggest that endogenous nitric oxide plays an important role in the gastroprotection of A. adstringens methanol extract on ethanol-induced gastric mucosal lesions and that there is partial participation by prostaglandins and endogenous sulfhydryls. The effect of 3alpha-hydroxymasticadienonic acid was attenuated only by pretreatment with N-ethylmaleimide, indicating that endogenous sulfhydryls (thiols) participate in its gastroprotective mechanism. Capsaicin-sensitive afferent neurons do not participate in the gastroprotection of either the methanol extract or 3alpha-hydroxymasticadienoic acid.