Diazepam-induced GH secretion was tested on 28 male volunteers before and after a 3-day treatment with methysergide, pimozide, or sodium valproate. Serum GH, diazepam, and blood glucose levels were determined. Without prior medication, the mean serum GH level increased 336% 1 h after diazepam administration. Treatment with the serotonin antagonist, methysergide, had no effect on the diazepam-stimulated GH secretion, whereas pimozide, the selective dopamine receptor-blocking agent, reduced the GH response to diazepam by 50% (P less than 0.05). Sodium valproate, a gamma-aminobutyric acid transaminase inhibitor, also inhibited diazepam-induced GH secretion; stimulated GH levels were 51% at 30 min (P less than 0.025), 39% at 60 min (P less than 0.025), and 46% at 90 min (P less than 0.025) relative to the stimulated levels without medication. No difference was found in blood glucose or serum diazepam levels after the drug treatments relative to the values obtained under basal conditions. It is suggested that diazepam-induced GH secretion is at least partly mediated via dopaminergic mechanisms. Serotonin does not seem to be involved. It is further proposed that gamma-aminobutyric acid plays an inhibitory role in GH secretion.