[ ABSTRACT] AIM:To investigate the preventive effects of Clostridium butyricum ( C.butyricum) on the type of pylorus ligated gastric ulcer ( GU) in mice and the underlying mechanisms.METHODS:ICR mice were randomly divided into 4 groups:sham operation group, model group, C.butyricum pretreatment group and omeprazole pretreatment group. Gastric pyloric ligation was adopted to establish GU model in mice.The gastric juice was collected to measure the content of gastric free mucus, the pH of gastric juice and the activity of pepsin.The gastric tissues were collected for routine HE stai-ning to observe the pathological changes.The content of glycogen was detected by PAS staining.The protein expression of Bax and Bcl-2 in the gastric mucosa was also assessed by immunohistochemical staining.RESULTS: The HE and PAS staining showed that the C.butyricum pretreatment obviously attenuated the mucosa lesion induced by ligation.Compared with model group, the pH of gastric juice was significantly raised.The activity of pepsin fell off in C.butyricum group, which was lower than that in omeprazole group.In comparison with model group, the content of gastric free mucus was dra-matically increased and PAS staining showed a significant rise in C.butyricum group, but not in omeprazole group.The protein expression of Bax was decreased and the protein expression of Bcl-2 was upgraded in C.butyricum group than those in model group.CONCLUSION:C.butyricum protects gastric mucosa against the challenge of pylorus ligation in mice and its mechanism may be related to inhibiting gastric acid secretion and the activation of pepsin, increasing the production of gastric free mucus, strengthening the expression of bcl-2 gene and inhibiting the expression of bax gene.