Hexarelin (His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH2) is a new synthetic growth hormone (GH)-releasing hexapeptide. The mechanism of action of hexarelin in man is not fully elucidated. As for other GH-releasing peptides, an action on both the pituitary gland and the hypothalamus has been hypothesized. In the present study, we evaluated the modulation of GH-releasing activity of hexarelin in man. In a first experiment conducted on 6 healthy male volunteers, we studied the interaction of the maximally effective intravenous dose of hexarelin (2 µg/kg i.v.) with GH-releasing hormone (GHRH, 2 µg/kg i.v.) and somatostatin (2 µg/kg/h i.v.). In a second experiment involving another 6 male subjects, we evaluated the interaction of hexarelin with neuroactive substances, such as pirenzepine (0.6 mg/kg i.v.), pyridostigmine (120 mg p.o.) and arginine (0.5 g/kg i.v.), thought to modulate endogenous somatostatin secretion. Hexarelin induced a higher increase in GH levels as compared to GHRH (integrated output calculated as area under the curve AUC0-120 4,693 ± 691 vs. 1,494 ± 102 µg·min/l p < 0.01). Coadministration of hexarelin and GHRH produced a higher GH response than hexarelin alone (AUCo-120 7,395 ± 450 µg·min/1, p < 0.05). Somatostatin abolished the GH response to GHRH (AUC0–120 363 ± 89 µg·min/1, p < 0.01), while it only blunted that to hexarelin (AUCo-120 1,314 ± 297µg·min/l, p < 0.05). Pirenzepine blunted the GH response to hexarelin (AUC0–120 1,931 ± 446 vs. 4,586 ± 674 µg·min/1, p < 0.05) while pyridostigmine and arginine did not modify the GH response to the hexapeptide (AUC0–120 5,179 ± 771 and 4,743 ± 774 µg·min/1). Thus, the GH-releasing activity of hexarelin is greater than that of GHRH and is even enhanced by the neurohormone. Hexarelin differs from GHRH in that its effect is only blunted by exogenous somatostatin or pirenzepine and is not potentiated by pyridostigmine or arginine. These results indicate that, in man, hexarelin and GHRH may have different mechanisms of action and that the potent GH-releasing activity of hexarelin is partially refractory