Summary. Expanded studies on the spontaneous diabetes mellitus in Macaca nigra provide additional support to analogies between this animal mode] and human diabetes. Abnormal signs include hyperglycemia, decreased clearance of glucose in intravenous tolerance tests, reduced insulin secretion and increased serum lipids (triglyceride, prebetalipoprotein and nonesterified fatty acids). Insulin secretory capacity is lost concomitant with amyloid infiltration into the islets of Langerhans; additional metabolic aberrations may also reduce insulin secretion or action. Secondary manifestations are atherosclerosis, thickened basement membranes of muscle capillaries, and cataracts. In all probability, a genetic predisposition in these monkeys is exacerbated by changes in diet and environment.