The serum GH response to oral L-Dopa during the separate infusions of saline, phentolamine and propranolol was examined in 12 acromegalic patients. In six L-Dopa responsive patients, serum GH concentrations rose during propranolol infusions (and fell during phentolamine infusions as previously reported). There was no effect of propranolol or phentolamine on the fall in serum GH following oral L-Dopa in this group of patients. Thus, β -adrenergic, as well as α-adrenergic, mechanisms are involved in the regulation of GH secretion in a qualitatively normal fashion in some acromegalic patients. However, the qualitatively abnormal suppressive effect of L-Dopa on GH secretion seen in some acromegalic patients does not appear to be mediated through either α-adrenergic or β-adrenergic mechanisms.