Oral 5-hydroxytryptophan (5-HTP) administration, either 200 mg acutely or 50 mg q.i.d. for 3 days plus 200 mg acutely, failed to modify either basal or TRH-stimulated prolactin secretion in normal subjects. This is at variance with the stimulatory action of intravenous tryptophan on human prolactin release. However, it is doubtful that 5-HTP at the dose used may increase brain serotonin concentration; moreover, the hydroxylated amino acid is also taken up by catecholaminergic neurons, from which noradrenaline and dopamine may then be released. It is concluded that the failure of 5-HTP to affect prolactin secretion in man is not a proof against the existence of serotoninergic stimulatory influences on human prolactin release.