The flowers and flower buds of Cistus laurifolius L. are used for the treatment of peptic ulcers. Through the bioassay-guided fractionation of the material, using water immersion and immobilisation-induced stress ulcer model, the EtOH-precipitated part from the aqueous extract (E-H2O down arrow) was determined to be the active fraction. For the evaluation of the mode of action, the activity of E-H2O down arrow was tested using various ulcer models in rats and mice and this fraction was found active against pylorus ligation-, abs. ethanol-, indomethacin-, indomethacin plus HCl/EtOH-induced gastric and cysteamine-induced duodenal lesions while ineffective against serotonin-induced gastric lesions. The active fraction showed its activity not only on per os administration but also after subcutanous injection. According to the results of biochemical studies, the active fraction showed a potent antiacid activity. In addition, histopathological, and toxicological studies were conducted with the active fraction. (C) 1997 Elsevier Science Ireland Ltd.