The effects of amitriptyline hydrochloride and desipramine hydrochloride treatment on brain serotonin (5-HT) function were investigated in 21 patients. The ability of an intravenous infusion of the serotonin precursor tryptophan to raise serum prolactin (PRL) levels was determined in 13 depressed patients during placebo administration and after 28 to 35 days of treatment with either amitriptyline or desipramine. Both desipramine (N = 7) and amitriptyline (N = 6) significantly increased the PRL rise induced by tryptophan compared with a preceding placebo period. In contrast, following long-term amitriptyline and desipramine treatment, the ability of tryptophan to increase PRL was enhanced two weeks following abrupt cessation of amitriptyline therapy (N = 5), but not after discontinuation of desipramine therapy. The results of this investigation are consistent with electrophysiologic and behavioral studies in laboratory rats and suggest that desipramine- and amitriptyline-induced alterations in 5-HT function may be related to their antidepressant mechanism of action.