The effects of intravenous infusion of the nonselective alpha-adrenergic antagonist phentolamine or of the selective alpha2-adrenergic antagonist yohimbine on growth hormone (GH), prolactin (PRL) and cortisol secretion during insulin-induced hypoglycemia were studied in 11 healthy young men. The GH response was blunted following each antagonist used, PRL secretion was higher after yohimbine and diminished after phentolamine when compared to controls. The plasma cortisol response was not influenced by either compound. In another series of experiments no effect of an oral administration of prazosin, a selective alphai-adrenergic antagonist, on the secretion of GH, PRL and cortisol was found in any of 7 subjects. Prazosin inhibited blood pressure increase during hypoglycemia and induced slight drowsiness and fatigue in the subjects. It is concluded that in man alpha-adrenergic stimulation of GH secretion during hypoglycemia is transmitted via alpha2-receptors, PRL secretion is mediated via alphai-receptors, whereas inhibition of PRL release is mediated via alpha2-receptors. In this experiment no effect of alpha<sub>1</sub>- or alpha<sub>2</sub>-blockade on cortisol response to hypoglycemia was seen.