The (3H) cyclic AMP accumulation was measured in incubations of pancreatic islets from one-day, six-day, and thirty-five-day-old rats exposed to a low (0.6 mg./ml.) or a high (3.0 mg./ml.) glucose concentration with or without the addition of 0.1 mM. of the phosphodiesterase inhibitor 3-isobutyl- 1 -methylxanthine (IBMX). In the thirty-five-day-old rats, (3H) cyclic AMP accumulation was significantly enhanced after sixty minutes' incubation in a high glucose concentration and further increased by IBMX. These changes were paralleled by a stimulated insulin release, measured simultaneously. By contrast, in the one-day-old rats, no effect of glucose with or without IBMX was seen on (3H) cyclic AMP, while the minor insulin release due to high glucose alone was markedly potentiated by IBMX. Even in the presence of this agent the insulin response to glucose was, however, clearly inferior to that seen in the thirty-five-day-old animals. The stimulatory patterns of glucose-induced insulin release in the six-day-old animals was intermediate between the other two age groups. At this age, stimulation of (3H) cyclic AMP due to glucose was observed only in the presence of IBMX. Measurement of (3H) cyclic AMP after three minutes' incubation confirmed these different stimulatory patterns of glucose in the age groups studied. It is suggested that the inefficiency of glucose to stimulate the adenyl cyclase-cyclic AMP system of the beta cell from fetal and neonatal animals may be one important factor determining the insensitivity to the insulin-releasing action of glucose that exists at this stage of development.