SUMMARY: The effect of reserpine on growth hormone (GH) secretion induced by insulin hypoglycaemia and by arginine infusion was studied in ten normal subjects and ten hyperthyroid patients. In both groups of subjects, oral reserpine administration resulted in a reduction of GH response to hypoglycaemia, but caused no significant change of GH release after arginine infusion. These results strongly support the theory of an adrenergic control of GH secretion and indicate that arginine stimulates GH secretion in a different way from insulin and independently of catecholamines.