1. Insulin secretion of individual isolated islets of Langerhans from mice, determined at 15 second intervals, demonstrated a pulsatile pattern. Glucose stimulated increased amounts of insulin to be released from these individual islets. 2. Insulin secretion of individual isolated mouse islets of Langerhans incubated for 5-15 minute intervals showed a direct relationship to concentration. There was negligible "leakage" in the absence of glucose. 3. Serial determination of insulin release from individual islets at 15 sec. intervals also demonstrated augmented magnitude of insulin release in response to increase of glucose in the medium. More important, a pulsatile pattern of insulin release was evident, which may be related to inherent physiological periodicity of islet physiology as also reflected in the pattern of islet electrical activity.