The islets of Langerhans in sections from the pancreas tail of Macaca nigra were stained by antiserum to insulin, glucagon, or somatostatin. The area of stained cells per total area of the islets was determined by a computerized photometric method. Insulin of the beta cells occupied 77% of the islet area in nondiabetic (ND) monkeys and decreased to 62% in monkeys in the earliest stages of metabolic deterioration, i.e., hormonally impaired (HI) monkeys. At the later stage of borderline diabetes (BD), monkeys had only 39% of the islet area occupied by insulin and the area was diminished to 1% in diabetic (D) monkeys. Islets in HI monkeys had an unusual pattern in which only the beta cells in the periphery of islets were stained. Glucagon in the alpha cells stained 7% of the islet area in ND monkeys, but the area was almost doubled to 13% in HI monkeys; the percentage decreased to about 5% in BD and 3% in D monkeys. Somatostatin accounted for 5% of the islet area in ND monkeys, was slightly greater at 7% in HI monkeys, and decreased to 3% in BD and 2% in D monkeys. Alterations in percentages of secretory cells correlated with several of the metabolic and clinical Changes.