Glycyrrhetinic acid (Ia) and eighteen related derivatives were examined for antiulcer activity using stress-induced gastric lesions (restraint plus water immersion at 25°C) in mice and rats as screening tests. Among the compounds tested, dihemiphthalate derivatives of 18α- or 18β-olean-12-ene-3β, 30-diol (IV, IIId), 18β-olean-9(11)12-diene-3β, 30-diol (VIc), and olean-11, 13(18)-diene-3β, 30-diol (VIIc) showed potent inhibition of gastric lesion formation at a dose of 12 or 25mg/kg (p.o.); carbenoxolone sodium (Ib) significantly suppressed the lesion formation at a dose of 500mg/kg (p.o.). Further evaluation of the antiulcer activity was carried out mainly for compound IIId. Compound IIId (p.o.) prevented the formation of indomethacin-induced or 0.6N HCl-induced gastric lesions; the latter antiulcer effect was noted even in the combined treatment with indomethacin, suggesting that the effect occurs independently of endogenous prostaglandins. In contrast, compound IIId had no preventive effect against Shay rat ulcer when intragastrically (i.g.) administered; further, no antisecretory effect was seen by i.g. application in pylorus-ligated rats. Administration of compound IIId for 2 weeks accelerated the healing rate of acetic acid-induced gastric ulcer in rats. No significant change in urine excretion was observed after its consecutive administration for 3d. These results suggest that dihemiphthalate derivatives (IIId, IV, VIc, VIIc) may produce a strong antiulcer activity, probably by strengthening some gastric mucosal defensive mechanism.