Glucose, apart from its acute insulin-releasing effect, exerts a time-dependent potentiating action on subsequent stimulations of the pancreas. The influence of a 24-hour starvation on these two actions of glucose was studied with the completely isolated, perfused rat pancreas preparation. Starvation had no effect on the time kinetics of the insulin response, but the magnitudes of the early and late insulin-secretion phases were reduced to similar extents. It was demonstrated by using a wide glucose concentration range that the maximal insulin response is not significantly modified by starvation. In contrast, both the threshold of stimulation by and the Km for glucose were higher in the pancrease from fasted rats. Thus, starvation reduces the sensitivity of the islet for the insulin-releasing action of glucose. When the stimulatory concentrations of glucose were preceded for 40 minutes by the perfusion of 8.3 mM instead of the basal, 4.4 mM glucose, insulin secretion from the pancreas of fed animals was not modified. In contrast, raising the glucose concentration of the equilibrium period to 8.3 mM potentiated markedly the insulin response to subsequent stimulations in the pancreas from fasted rats. This potentiation expressed itself as increase in the maximal response: the Km for glucose was not reduced. Thus a 40-minute pretreatment with 8.3 mM glucose does not correct the diminished sensitivity induced by a 24-hour starvation. It is concluded that in starvation (1) the sensitivity for glucose of the mechanisms that initiate insulin release is diminished; (2) the sensitivity of the pancreas for the potentiation-inducing action of glucose is augumented. (3) In both respects, the insulin response of fasted rats is similar to that of mildly diabetic subjects. These and other findings suggest that the effect of glucose in initiation of insulin release and on generation of a state of potentiation in the islet are mediated by different mechanisms.