The aim of the present study was to investigate the proliferation of the epithelial cell in the stomach and the intestine of newborn and adult horses. For the first time the Ki-67 immunohistochemistry was applied in this study. Special interest was to see if there would be tissue-specific or regional differences comparing the gastrointestinal epithelium of foals and adults. The defined epithelial tissue specimens (n=14) were taken out of each of 11 foals and 7 adults stomach and intestine directly after animals were killed. After that the epithelial specimen were prepared for lightmicroscopical and following immunohistochemical investigation. The monoclonal MIB-1 an anti-Ki-67-antibody was used to detect Ki-67. The part of the Ki-67 positive cells is given by the proliferation index (Pi) in percent. It could be proven that it is possible to determine proliferating activity with Ki-67 on gastrointestinal tissues in horses. The results show that there are significant differences concerning the proliferation in the pars glandularis of the stomach. Comparing the proliferation rate in foals to adults in this part the newborns show less proliferation: Fundus Pi: foals 9,43 % adults 16,95 % p=0,0016 Margo plicatus Pi: foals 19.96 % adults 32,72 p=0,003 Pylorus Pi: foals 11,42 adults 21,25 p=0.007 There is no significant difference in the pars nonglandularis. The main part of the proliferating cells were localized in the isthmus region in the pars glandularis of the stomach. Maybe the smaller proliferation rate in the pars glandularis of the stomach in foals is related to a higher lifespan of epithelial cells and different adaptative processes. In the small and in the large intestine both age groups show no visible differences in the average proliferation rate: Small intestine: Pi: foals 31,22 % adults 31,88 % Large intestine: Pi : foals 24,20% adults 24,99 % In contrast to the distribution pattern in the stomach the intestine cells proliferated most extensively in the crypt area to the bottom of the tunica mucosa. It could be suggested that besides proliferation-stimulating factors also inhibitoric factors affect the proliferation of the intestinal epithelial cells in foals. This could be the reason why the proliferation rate of the foals is close to the one of the adult horses. Considering gastrointestinal diseases in horses, Ki-67 could become more important in future. Primarily neoplasias in the gastrointestinal tract and in postoperative prognostics.