5-Hydroxytryptophan (5-HTP) was found to lower plasma 11-oxycorticoids and increase plasma growth hormone in anesthetized dogs. To determine whether these effects were mediated by release of serotonin, 5-HTP and L-tryptophan were injected intravenously alone and after treatment with various drugs. The decrease in corticoids produced by 5-HTP was unaffected by intravenous carbidopa and metergoline, but was abolished by intravenous benserazide in a dose that inhibits central 5-HTP and dopa decarboxylase, and by a small dose of phenoxybenzamine in the third ventricle. Adrenal sensitivity to ACTH was unaffected by 5-HTP. L-Tryptophan did not decrease plasma corticoids; instead it produced a small increase. The increase in plasma growth hormone produced by 5-HTP was potentiated by carbidopa and abolished by benserazide. L-Tryptophan did not increase plasma growth hormone; instead it produced no change or in some instances a decrease. Plasma growth hormone declined after administration of phenoxybenzamine in the third ventricle, and the decline continued after administration of 5-HTP. The data indicate that the effects of 5-HTP on ACTH and growth hormone secretion are not due to central release of serotonin and suggest that they are due instead to serotonin-induced release of catecholamines from catecholamine-secreting neurons.