AIM: To examine the changes in the histomorphology of the gastric, jejunal and colonic wall of dogs due to physiological aging.METHODS: Full thickness biopsies were taken from the gastrointestinal tracts of 28 dogs of different ages.The thickness of the different layers of the wall was measured and the numbers of proliferating cells as indicated by immunohistochemical detection of Ki67 were counted.RESULTS: In the three excision sites, the thickness of all subepithelial layers increased with rising age. The strongest correlation between age and thickness of the intestinal wall was found in the first 10 years of life and in the jejunum (r = 0.6-0.71 for the deep lamina propria mucosa, the muscularis mucosa, and the circular layer of the tunica muscularis). The number of proliferating cells decreased during aging, with the strongest correlation in the lamina propria mucosa and lamina muscularis mucosa of the jejunum and in the colonic submucosa (r= -0.61 to -0.71). Epithelial proliferation was only weakly correlated to the age.CONCLUSION: The morphology of the deeper layers and the proliferation of mesenchymal cells of the intestinal wall of healthy dogs are correlated with age.Gastrointestinal epithelial proliferation is only weakly age-correlated.