The thickness of the capillary basement membrane of femoral muscle was examined in normal and spontaneously diabetic Macaca nigra. Thickness correlated significantly with the degree and severity of diabetes; greater thickness was associated with decreased glucose tolerance, decreased serum insulin, and increased glucose and triglyceride. In normal monkeys, the average basement membrane thickness was 696 Å and in diabetic monkeys, 837 Å. The minimum basement membrane thickness was 524 Å in normal monkeys and 634 Å in diabetics. Normals differed significantly from diabetic monkeys.