Abstract: Diabetes mellitus causes some changes in urine quality and quantity. Therefore, we planned this study to explain how these changes affect the histological structures in the urinary bladder. We classified this experimental subjects under 4 Groups as follows; Group I: Control group, Group II: 60 mg kg-1 single dose of streptozotocin was administered intraperitoneally (i.p.), Group III: 40 mg kg-1 dose of streptozotocin i.p. was administered 5 days consecutively, Group IV: 150 mg kg-1 single dose of alloxan was administered i.p. We obtained the biopsy materials of tunica mucosa of the urinary bladder after 6 months, then we exposed these materials for observations with light and electron microscopy. In Group II, we observed a decrease in the number of urothelium layers, loss of dome shaped cells which were replaced with the intermediate layer cells. In Group II and especially in Group III, we observed an increase in the thickness of the epithelial and capillary basement membranes. In Group IV, we observed a repair process in urothelium. The increase in the thicknesses of the basement membranes were less than the other groups. After the 6 months of experimental period, the degeneration of tunica mucosa in Group III, which had the highest blood glucose levels, seems to be related to the complications of lasting diabetes. The mild degeneration of tunica mucosa in Group II and Group IV seems to be related to the insuline production which was caused by β cell regeneration or island cell adenoma and also these factors reduced the complications of diabetes seriously.