In seven normal subjects the repeated oral administration of metergoline, a specific antiserotonin agent, has enhanced HGH response to arginine infusion. Following placebo administration, arginine-induced HGH release was slightly but not significantly reduced; similarly, in eight metergoline treated subjects, HGH response to oral L-Dopa was slightly but not significantly reduced. HGH response to i.v. L-Dopa was not modified by the drug. These results suggest that serotonin controls HGH response only in response to arginine, not to L-Dopa.