In order to determine whether or not a serotoninergic mechanism is involved in the secretion of the growth hormone (GH), a study was made of the effect of cyproheptadine, an antiserotonin agent, on plasma GH levels in normal children. The oral administration of cyproheptadine (12 mg daily) reduced GH responses to both L-dopa and glucagon; similarly, the mean GH response to oral glucose administration was significantly reduced after cyproheptadine administration. Plasma glucose values during the tests were not altered by cyproheptadine therapy. These results suggest that the serotoninergic mechanism might be involved in L-dopa-, glucagon-, and glucose-induced GH release.