Sex differences in the neuroregulation of GH secretion are not now known in humans. To investigate whether activation of cholinergic tone by pyridostigmine could cause a sex-related difference in the pituitary responsiveness to GH-releasing hormone (GHRH), we have studied the GH response to GHRH in 16 normal subjects (8 men and 8 women) tested after oral placebo or different doses of pyridostigmine (30, 60, and 120 mg). Each subject presented a normal response after iv administration of 50 micrograms GHRH and placebo. In men each dose of pyridostigmine induced a significant increase in the GH response to GHRH, as assessed by both the maximal GH peak and the area under GH curve. In women, on the contrary, the GH response to GHRH was not potentiated by pretreatment with pyridostigmine at any given dose. Only five female subjects were tested with 120 mg pyridostigmine because of the severe side-effects of the drug at this dosage. Our present data strongly suggest that in humans there is a sex-related difference in the neuroregulation of GH secretion and this is probably expressed through a different cholinergic tone.