Absent prolactin response to l-tryptophan in normal and acromegalic subjects

l-tryptophan (5 g orally), the precursor of serotonin, was administered to 10 patients with acromegaly and eight control subjects. In normal subjects, there was no significant change in serum prolactin after l-tryptophan. In subjects with acromegaly, there was no significant net change in serum prolactin after l-tryptophan relative either to the baseline value before l-tryptophan or to values obtained on a control day without l-tryptophan. Activation of serotoninergic pathways by oral administration of the serotonin precursor l-tryptophan has no effect on serum prolactin in normal or acromegalic subjects.