"Bile reflux" is a common term to denote a process of placing duodenal contents in the stomach and/or lower oesophagus. It is most often associated with functional or organic failure of the pylorus and is a not uncommon postoperative condition after pyloric section, resection or by-passing. Gastrotoxicity of the replaced small intestinal mixture leading to lesions in gastric mucosal barrier, is caused by an increased ability to reabsorb hydrogen ions along with migration of blood proteins and electrolytes towards lumen of the stomach. Consequently, histamine secretion becomes increased, leading to inflammatory and haemorrhagic changes or ulcer niches. The aim of the study was to demonstrate histological and microscopic changes in the gastric mucosa following reflux and to determine if long-term exposure to refluxed duodenal contents will produce tumorous changes in the organs tested. Material and methods. The study consisted of 25 mature female Wistar rats weighing 180-200 g. Bile reflux to the stomach was produced experimentally by surgical drainage. Final evaluation was performed after 55 weeks. Results. Findings were as follows: gastric changes were noted in basal and parietal cells, no tumorous foci were found in histological samples. Slight morphological changes can be caused by short periods of gastric mucosa exposure to the gastrotoxic small intestinal mixture. Conclusions. Endogenous bile acids cause morphological changes in the stomach mucosa of rats. In particular, these changes affect the ultrastructure of basal and parietal cells. No neoplastic foci were found in the examined organs.