Ghrelin is a novel growth hormone (GH) releaser acylated peptide that has recently been purified from stomach, and which potently binds to the GH secretagogue receptor. Ghrelin releases GH in vitro and in vivo in animal models, however its actions, potency and specificity in humans are unknown. In the present study, 12 healthy subjects were studied: 6 underwent four tests with ghrelin administered i.v. at the dose of 0 (placebo), 0.25, 0.5 and 1 microg/kg which corresponds to 0, 18, 37 and 75 microg total dose. A further 6 volunteers underwent two tests on different days with ghrelin at the dose of 3.3 or 6.6 microg/kg which corresponds to 250 microg and 500 microg total dose. Ghrelin-mediated GH secretion showed a dose-response curve, in which 1 microg/kg was the minimally effective dose in some individuals, but not as a group. On the contrary, the total doses of 250 microg and 500 microg elicited a powerful GH secretion, with a mean peak of 69.8+/-9.2 microg/l and 90.9+/-16.9 microg/l respectively, and areas under the curve of 4435+/-608 and 6125+/-1008 microg/l per 120 min respectively. All of them statistically significant vs placebo and vs the 1 microg/kg dose. Ghrelin administration also elicited a relevant dose-response mediated prolactin secretion suggesting no specificity of its actions. No relevant side effects were observed with ghrelin apart from a hyperhydrosis episode in two individuals tested with the higher ghrelin doses. In conclusion, ghrelin is a potent releaser of GH in normal individuals, with a dose-response pattern of operation. No saturating dose was observed.