We studied the microvasculature of the pancreatic islet with an electron microscope using 15 alloxan-diabetic and 16 control rats. These animals were morphologically examined at 2 weeks, 1, 2, and 4 months after alloxan injection (diabetic rats) or at comparable times (control rats). Control rats were non-diabetic, either sequentially treated with glucose and alloxan (injected controls) or not subjected to any medication (non-treated controls). Body weights, plasma glucose levels, and erythrocyte glycosylated hemoglobin (HbA1c) concentrations were also measured at intervals. All the diabetic rats exhibited retarded growth, hyperglycemia, and increases in HbA1c concentrations. The primary changes in diabetic islet capillaries can be grouped into four categories: (1) narrowing and closing of capillaries and occurrence of possible acellular capillaries, (2) formation of perivascular edema, (3) bulging and projection of endothelial cytoplasm, and (4) perivascular proliferation of collagen-like fibrils together with thickening of the basement membrane. These changes tended to worsen with diabetic duration. They are regarded as pathognomonic for diabetic microangiopathy. It is presumed that the present microvascular lesions of the islet play an important role in the pathogenesis and advancement of the diabetic state.