The rates of oxygen consumption and glucose oxidation were measured in isolated pancreatic islets of obese-hyperglycemic or normal mice. Sufficient analytical sensitivity was achieved by incubating islets in Cartesian divers, allowing accurate measurements of oxygen uptake in the range of 1 to 10 mug dry islet weight. The endogenous islet respiration proceeded at a constant rate for more than one hour. Glucose effected a rapid and lasting elevation of the respiratory rate which corresponded to the observed rate of glucose oxidation. An extended period of starvation led to a reduction in both oxygen uptake and glucose oxidation. Isolated islets maintained for seven days in tissue culture at a glucose concentration of 5.5 mM showed a pronounced increase in oxygen consumption when they were incubated with the nonmetabolizable insulin secretagogue b (+/-) BCH in the absence of glucose. The respiratory stimulation was less marked after culture at 3.3 mM glucose and only faint after culture at 16.7 mM glucose. A common feature of islets exposed to BCH in the absence of glucose was a gradual decline of respiration commencing 30 to 40 minutes after addition of BCH. Islets incubated with BCH in the presence of glucose displayed, however, no respiratory retardation. BCH produced only a weak stimulation of the islet glucose oxidation. It is concluded that BCH stimulates the endogenous substrate oxidation of the B-cells, possibly by affecting a noncarbohydrate pool of substrate. The observations add further support to the idea that the regulation of insulin release is linked to metabolic events in the B-cell.