Administration of ACTH beta 1-24 (0.25 mg, i.m.) in six healthy subjects reduces blood prolactin response to sulpiride (100 mg, i.m.). Dexamethasone (2 mg, orally, given at 11.00 p.m. of preceding day) blocks blood prolactin response to insulin-induced hypoglycemia (0.20 U. kg of "Actrapid" Novo, i.v.) in six voluntary healthy subjects. Intravenous naloxone (0.4 mg/Kg of body weight) blocks in man blood prolactin response to insulin-induced hypoglycemia. On the basis of the above data we can hypothesize that ACTH and dexamethasone have influenced by a feed-back mechanism the lypocorticotropic system. Such system by beta-endorphin release can modulate the secretion of prolactin. Thus inhibition of the lipocorticotropic system may be responsible for the reduced response of the prolactin system to stimulation by sulpiride or hypoglycemic stress.