Arginine stimulation test was performed on 36 healthy volunteers from both sexes, in order to determine whether alpha-adrenergic histaminergic or serotoninergic mechanisms was involved in growth hormone (GH) secretion. Arginine alone (30 gr/30 min) induced a clearcut GH response significantly different versus basal values in two different control groups. This response was accompanied by a slight, statistically non-significant, decrease in glucose and FFA levels. The concomitant administration of the alpha-adrenergic blocker phentolamine (0.5 mg/min x 90 min) did not alter the GH secretory pattern elicited by arginine. The addition of the H-1 histaminergic blocker diphenhydramine (50 mg i.v. as bolus 15 min before and 50 mg infused with arginine) completely suppressed the arginine induced GH secretion. Diphenhydramine, was also able to reduce GH basal values. Almost identical results were obtained with cyproheptadine, a serotoninergic, histaminergic and cholinergic blocker agent (2 mg q.i.d.; per os x 2 days and 4 mg per os 120 min before arginine). This drug suppressed arginine induced GH secretion without altering basal GH values. Blood glucose or FFA levels were not altered by any pharmacological manipulation vs. the control arginine group. It is concluded that either histaminergic H-1 or both histaminergic and serotoninergic blockade inhibit GH secretion elicited by arginine. On the other hand an alpha-adrenergic pathway in the arginine action is ruled out.