The present study provides evidence that changes in glucagon secretion are influential in determining the hyperglycemic activity of catecholamines in normal and diabetic rats. In normal fed rats, epinephrine (EPI) stimulated large increments in glucagon release and inhibited insulin secretion. In contrast, the modest hyperglycemic activity of isoproterenol (ISO) in normal fed rats correlated with its weak glucagon-releasing activity and its strong insulin-releasing activity. In alloxan-diabetic rats, the augmented hyperglycemic response to ISO was accompanied by larger increments in plasma glucagon levels than the catecholamine produced in normal fed rats. When glucagon release was inhibited by a concurrent infusion of somatostatin, the hyperglycemic responses of normal fed rats to EPI were reduced by approximately 67%. ISO-induced hyperglycemia in alloxan-diabetic rats was even more sensitive to inhibition by somatostatin since this response was reduced by approximately 90% when glucagon release was inhibited by somatostatin. These findings indicate that more than half of the hyperglycemic response to EPI in normal fed rats and nearly all of the hyperglycemia produced by ISO in diabetic rats result from increased glucagon release. Moreover, the impotence of ISO as a hyperglycemic agent in normal fed rats is probably due to insulin release which tends to suppress glucagon release.