Plasma levels of GH and prolactin were measured by radioimmunoassay in male domestic fowl treated with centrally active agents. p-Chlorophenylalanine (pCPA) did not have an effect on tonic levels of prolactin but led to a significant rise in circulating GH concentrations. The three serotonin receptor antagonists tested, methysergide, SQ-10631 and cyproheptadine, each resulted in a significant reduction in plasma prolactin while markedly increasing plasma GH levels. Administration of 5-hydroxytryptophan led to a rise in plasma prolactin and a drop in plasma GH levels in untreated birds or in animals pretreated with pCPA. The serotonin receptor agonist, quipazine, resulted in a marked increase in plasma prolactin and a marked reduction in plasma GH concentrations in untreated birds. In pCPA-pretreated animals quipazine was no longer effective in altering plasma prolactin levels but still caused a significant drop in circulating levels of GH. These results suggest that in the young male domestic fowl serotonin has a stimulatory role in the regulation of prolactin and an inhibitory role in the regulation of GH secretion.