It is known that streptozotocin (STZ)-induced diabetes causes functional and structural alterations in some types of tissue and organ. A number of methods have been used to characterize the properties of diabetic tissues and their diagnosis. Selenium compounds, playing an antioxidant role, can restore some altered metabolic parameters and diminished functions in experimental diabetes. The first aim of the present study was to investigate the effects of STZ-induced diabetes on structural properties of rat long bones. Electron and light microscopic observations showed deleterious alterations in the structure of the diabetic rat long bones, the most prominent effect being in osteocytic cells. Fine cytoplasmic processes of the osteocytes seemed to be shortened, and diabetes affected the normal cytoplasmic processes in a negative manner. The second aim of the present study was to evaluate the effects of sodium selenite treatment for 4 wk on the long bones of the diabetic rats. Electron and light microscopic observations demonstrated that sodium selenite treatment prevented the STZ-induced structural as well as ultrastructural changes in the long bones of the rats. In conclusion, this study first showed that a period of 5-wk diabetes was enough to cause some important and degenerative changes in the structure of the bone tissues, and, second, it demonstrated that sodium selenite treatment of the diabetic rats could normalize these alterations.