Glucagon release from isolated islets (collagenase digestion) in control and streptozotocin-treated diabetic rats was measured in varying glucose concentrations and in the presence or absence of added insulin. High glucagon release from diabetic islets was noted, and this glucagon release could not be suppressed by glucose. Incubation of diabetic islets with insulin, 1,000 μU./1 ml., suppressed glucagon release at high glucose concentrations. Addition of insulin to control islets had no effect, nor had the administration of insulin to diabetic animals before decapitation. It is concluded that insulin deficiency removes the normal glucose suppression of glucagon release and that the effect can be reversed by high concentrations of insulin.