AIM:To evaluate the changes of lung morphologic features and oxidative stress in experimental diabetic rats. METHODS: The pulmonary structure of alloxan-induced diabetic rats were quantitatively studied with stereological methods. Changes of ultrastructure, activities of superoxide dismutase (SOD) and contents of malondialdehyde(MDA) of diabetic lung and serum were observed. RESULTS: The volume proportion of alveolar air and mean linear intercept of diabetic rats decreased remarkably in comparison with controls while the volume proportion of alveolar wall, the surface density of alveolar, the numerical density of alveolic area, the numerical density of alveolar and specific surface of alveolar increased significantly. The major change of the type II pneumocyte of diabetic rats was dilation of the rough endoplasmic reticulum (RER). The other findings in diabetic rats had included the presence of thickened alveolar epithelial, pulmonary capillary basal laminae and blood- air barrier, the volume density, the surface density, the mean profile area and the mean perimeter of RER in type Ⅱ pneumocyte of diabetic rats increased remarkably. And the specific surface of RER was significantly lower as compared with controls. SOD activity decreased and MDA content increased significantly in serum of diabetic rats as compared with the control group. SOD activity in the diabetic lung was not different from that of the control lung. However, the content of MDA obviously increased in diabetic lung. CONCLUSION: The morphologic features and oxidative stress in early diabetic rats are abnormal thus the lung should be considered as one of the “target organ”in diabetes mellitus.