Studies were performed in healthy subjects to ascertain the neurotransmitter systems involved in the growth hormone (GH)-releasing effect of the potent enkephalin analog FK 33-824. Concomitant evaluation of prolactin (PRL) secretion was also performed in the same subjects. FK 33-824 at a dose of 0.5 mg IV elicited a clear-cut rise in plasma GH and PRL concentrations with peak levels at 45 min. Blockade of muscarinic cholinergic receptors by atropine (0.5 mg SC) or histaminergic H₁ receptors by diphenhydramine (50 mg IV bolus plus 50 mg infusion) completely suppressed the GH release induced by FK 33-824, without significantly altering the PRL rise induced by the peptide. Pretreatment with the alpha-adrenergic antagonist phentolamine (0.5 mg IV/min for 120 min) or the dopamine receptor blocker metoclopramide (10 mg IV) did not alter the GH-releasing effect of FK 33-824. Phentolamine failed to alter the PRL rise induced by FK 33-824, while combined FK 33-824-metoclopramide administration induced a greater PRL increase than FK 33-824 alone. These results indicate that cholinergic and histaminergic H₁ receptors play an important role in the GH-release induced by FK 33-824 in man, whereas this action seems to occur independently of catecholaminergic mediation. The same receptors are not involved in the PRL-releasing effect of the peptide.