A chronic diabetic state was produced in Macaca fuscatus , and these diabetic monkeys were kept without insulin treatment for up to 25 mo. The metabolic derangements were characterized by hyperglycemia, insulinopenia, hyperglucagonemia, ketonemia, and hyperlipidemia. Significant thickening of the capillary basement membrane of the gastrocnemius muscle was observed in the chronically diabetic monkeys, and became obvious in the course of diabetic state; 732 +/- 35 A in controls, 750 +/- 58 A in diabetic monkeys with duration of 4 mo, and 1165 +/- 112 A in those with duration of more than 11 mo. In addition to duration of the diabetic state, severity of hyperglycemia is also thought to play an important role in the capillary basement membrane thickening judging from the fact that diabetic monkeys with constant hyperglycemia showed a greater membrane thickening. Ultrastructural alterations, such as significant thickening of glomerular basement membrane and increase of mesangial matrix, were observed in kidney as well. These results indicate that diabetic microangiopathy has been produced by metabolic derangements characterized by chronic hyperglycemia, insulinopenia, and hyperlipidemia.