The ability of 5-hydroxy-L-tryptophan (5-HTP) or pentobarbital anesthesia to elevate rat serum GH levels is completely blocked by the simultaneous administration of the serotonin antagonist cyproheptadine, as well as by the pineal gland principles melatonin and 5-methoxytryptamine (5-MT), and by the O-methylated dopamine derivative 3,4-dimethoxyphenylethylamine (DMPEA). Small doses of 5-MT cause paradoxical elevations in serum rat GH but at the same time inhibit the action of 5-HTP. Blockade of dopaminergic pathways by treatment with either DMPEA or chlorpromazine causes a slight, nonsignificant suppression of serum GH in normal rats while serum prolactin levels are increased greater than tenfold, showing that, unlike prolactin, rat GH is not subject to tonic inhibitory control by dopamine. The data presented support the recently advanced hypothesis that rat GH is under serotoninergic control and demonstrate that GH secretion can be significantly suppressed by serotoninergic blockade. It is suggested that the results presented here for monoamine control of rat GH release are not at variance with those reported for man.