In diabetics two different patterns of insulin release were demonstrated. One type was found after the injection of an alpha-adrenergic blocking agent, phentolamine, another after injection of secretin. The insulin release after phentolamine was moderate and prolonged, while after secretin it was rapid, short lasting, and high. The insulin pattern after secretin was not affected by a preceding phentolamine injection. Probably different mechanisms are involved in these two types of insulin release.