The administration of glucose, tolbutamide, corticotropin, and glucagon to mice resulted in stimulation of insulin secretion. These stimulations were all inhibited by the prior administration of β-adrenergic receptor blocking agents. Cyclic 3',5' adenosine monophosphate stimulated insulin secretion in intact mice and its stimulatory effect was also inhibited by β-adrenergic receptor blocking agents. The locus of the inhibitory effects of β-adrenergic receptor blocking agents on insulin secretion is discussed.