The role of calcium in glucoseinduced insulin secretion was examined by measuring 45Ca uptake by isolated islets of Langerhans. The islets were incubated for 90 min in the presence of 45Ca, submitted to successive washings, and finally examined for their radioactive content. Glucose, but not galactose, stimulated 45Ca uptake by the islets. A sigmoidal curve characterized the relationship between 45Ca uptake and the glucose concentration of the incubation medium. Glucose-induced calcium uptake was increased in media in which Na+ had been partially replaced by K+, and inhibited by mannoheptulose, 2-deoxyglucose, epinephrine, diazoxide, low temperature, low pH, high Mg++ concentration, low Ca++ concentration, and in media in which Na+ had been partially replaced by Li+. Under these experimental conditions, the, changes in 45Ca uptake paralleled those in insulin secretion by the islets. However, although glucose failed to induce insulin release at low Ca++ concentration, the reduced uptake of calcium found under this experimental condition was still markedly decreased by the addition of mannoheptulose to the incubation medium. Thus, the rate of calcium uptake is not merely a function of the rate of insulin secretion. The data rather suggest that glucose, when metabolized within the beta-cell, stimulates calcium uptake, which in turn triggers the release of insulin. (Endocrinology88: 72, 1971)