Glucose intolerance was found in four adult chimpanzees. The response of glucose, insulin, C-peptide, and glucagon to intravenous glucose and tolbutamide stimulations revealed impaired glucose clearance, deficient pancreatic secretion of insulin and C-peptide, and elevated glucagon levels. Pancreatic islets in a diabetic chimpanzee were hypercellular, possibly due to α-cells. Minimal or no insulin was observed in β-cells. Results are consistent with the occurrence of noninsulin-dependent diabetes mellitus, which may be more prevalent in chimpanzees than heretofore suspected.