Time course of the changes in insulin release and cyclic (c) AMP levels in isolated rat islets incubated in media containing 5 or 16.7 mM of glucose were followed. The higher glucose concentration caused a slight but significant increase of cAMP levels after 10 min incubation, but not 5 min incubation; the stimulation of insulin release by 16.7 mM of glucose was apparent at both incubation times. Theophylline increased cAMP levels markedly but did not stimulate insulin release when the glucose concentration was 5 mM. A slight augmentation by theophylline of insulin release was observed in the incubation medium containing 16.7 mM glucose. The elevation of cAMP in islets may not play a role in the initiation of the insulin release induced by glucose, but it may act to modulate the glucose effect.