In normal women, intravenous injection of the H2-antihistamine, cimetidine, provoked a 3-4 fold rise in serum prolactin, without changes in serum growth hormone, thyrotrophin, or gonadotrophins. Hyperprolactinaemic patients with pituitary tumours, idiopathic hyperprolactinaemia or hypothalamic lesions demonstrated little or no rise in serum prolactin (expressed as a percentage increment) in response to cimetidine; these responses were significantly more blunted than the prolactin responses to intravenous TRH in the same subjects. Post-partum women also demonstrated blunted percentage prolactin responses to cimetidine, although responses to TRH were, in most patients, normal. Dynamic testing of prolactin secretion with cimetidine is no more useful than TRH in distinguishing tumourous from non-tumourous hyperprolactinaemia.