Abstract 1. 1. Cartesian divers of the ampulla type were used in measurements of the oxygen uptake of individual islets of Langerhans separated from pancreas of normal mice by a collagenase method. 2. 2. Incubated in a phosphate buffer supplemented with glucose the islets respired at a constant rate for several hours. The endogenous respiratory rate was constant for about 1.5 but then declined rather rapidly. In the first hour it amounted to 14.2·10 −4 μl/h per islet or 94 μmoles/h per g dry weight. Mannoheptulose did not inhibit endogenous respiration. 3. 3. 16.7 mM glucose stimulated islet oxygen uptake by 154% and this stimulation was completed abolished by 28.6 mM mannoheptulose. 4. 4. 10 mM pyruvate stimulated islet oxygen uptake by 55% and this stimulation was also abolished by mannoheptulose. The decarboxylation of [ I- 14C]pyruvate by islets incubated in a bicarbonate buffer was inhibited 71% by mannoheptulose. 5. 5. Raising the extracellular concentration of Ca 2+ from 0.9 to 2.5 mM did not affect the oxygen uptake in the presence of high glucose concentrations. 6. 6. The dry weight of islets of the size used in these experiments was 0.67 ± 0.02 μg/islet (mean ± S.E.). DNA constituted 2.4% of the dry weight and it was calculated that one medium-sized islet contains approximately 2200 cells. 7. 7. The results are discussed with reference to the diabetogenic action of mannoheptulose on whole animals and the interaction of mannoheptulose with stimulus-secretion coupling in the β-cells of pancreatic islets.