The role of alpha-adrenergic and serotoninergic systems in the glucagon induced growth hormone (GH) release, has been investigated in 25 normal volunteers of both sexes divided into four groups. Group I received only glucagon (Gn) 1 mg s.c.; Group II received after Gn an phentolamine infusion over 150 min at the rate of 0.5 mg/min; Group III 30 min after Gn received fenfluramine 20 mg as a bolus injection, followed by another 20 mg infused over 90 min period; and Group IV after two days of treatment with cyproheptadine (2 mg p.o., q i d) received the last dose of 4 mg, 120 min before Gn administration. Gn elicited a clear-cut GH release in the four groups irrespective of treatment. GH peaked at 150 min and this elevation was significantly different from the basal values. No significant differences were present among the groups regarding GH, glucose or free fatty acid levels. These results suggest that the alpha-adrenergic and serotoninergic systems are not involved in the Gn induced GH release.