The plasma c-AMP level increases significantly in response to pharmacologica doses (1 mg i. v.) of glucaton. Beta blocking agents somewhat inhibit this increase in both euthyroid and hyperthyroid subjects. Catecholamine release elicited by glucagon thus plays some part in the response. In euthyroid subjects 2 microgram/kg glucagon still produces a significant increase in the plasma c-AMP concentration and propranolol fails to counteract this response. The c-AMP response elicited by identical doses of glucagon is more marked in hyperthyroid than in euthyroid subjects and can be somewhat reduced by propranolol. This indicates that in hyperthyroid patients even low doses of glucagon stimulate the release of catecholamines or enhance the responsiveness of beta receptors. The plasma c-GMP level increases in response to glucagon loading but in a lesser degree than does the c-AMP concentration. The response of c-GMP to glucagon is more protracted than the response of c-AMP and propranolol fails to counteract it.