Starvation of rats for 24 hrs resulted in decreased insulin release from the isolated rat pancreas. The effect of fasting could not be counteracted by elevation of the glucose level in the equilibration medium from 0.8 to 1.5 mg/ml. The alpha-adrenergic blocking agent phentolamine (10 mug/ml) stimulated glucose induced insulin release to approximately the same extent in fasted as in fed rats. These findings illustrate the importance of endogenous catecholamines in the regulation of insulin secretion from the isolated pancreas. Our experiments suggest that the impairment of insulin secretion on fasting is due neither to the inhibitory effect of catecholamines nor to the lack of substrate in the pancreas at the initiation of the stimulation.