In 10 acromegalic patients mean (±SE) serum growth hormone concentrations fell to 67 ± 6% of mean base line (p < 0.01) after 2 hr of alpha-adrenergic blockade with phentolamine and to 50 ± 8% of mean base line (p < 0.001) 1 hr after the superimposition of beta-adrenergic stimulation with isoproterenol upon alpha blockade. Absolute serum GH levels fell significantly during phentolamine alone in 7 of 10 patients and during phentolamine- isoproterenol in 8 of 10 patients and were more than two standard deviations below the 24-hr GH profile mean, determined separately for each patient, in 4 of 10 patients during phentolamine alone and in 6 of 10 patients during phentolamineisoproterenol. This phentolamine-isoproterenol regimen did not accelerate the disappearance of intravenously injected human growth hormone. After oral L-dopa mean serum GH levels fell to 64 ± 10% of base line (p < 0.01) in these acromegalic patients. Absolute serum GH levels were more than 2 SD below the 24-hr GH profile mean in 5 of 10 patients after L-Dopa. The suppressive GH responses to fixed doses of phentolamine-isoproterenol and of L-dopa were inversely correlated with body weight. Thus, alpha-adrenergic blockade and superimposed beta-adrenergic stimulation are capable of suppressing GH secretion in acromegalic patients. These observations suggest that alpha-adrenergic mechanisms contribute to the sustained hypersecretion of GH in acromegaly and raise the possibility that diminished beta-adrenergic activity may also play a role in excessive GH secretion.