The early phase of the insulin secretory response to glucose may not be derived from the total pool of stored insulin, but possibly from a specific and relatively small fraction in a state or locus necessary for its immediate release. In an attempt to deplete this hypothetical fraction, and thereby to support the concept of its existence and obtain an estimate of its size under different circumstances, normal and mildly diabetic subjects were given a series of three large glucose loads followed by glucagon, with and without alcohol “priming”. Results indicate that repetitive glucose or glucagon challenges over a two-hour period failed to exhaust the supply of immediately releasable insulin in either group of subjects as judged by the increments in plasma insulin during the first few minutes after each challenge. Alcohol pretreatment resulted in a larger early insulin secretory response to the first of three glucose loads, but a decreasing response to the subsequent two, possibly indicating that the initially augmented secretory response had resulted in depletion of a finite supply of immediately available insulin. An unexpected finding was the inhibitory effect of alcohol on glucagon-induced insulin release, in direct contrast to its effect when glucose is the insulinogogue, suggesting that glucose and glucagon exert their effects throughdifferent mechanisms or on different fractions of the insulin pool.