In order to evaluate the interference of prolactin (PRL) on some aspects of pituitary-gonadal derangement observed in patients with hyperprolactinaemic amenorrhoea (HA), a GnRH stimulation test (100 micrograms i.v.) was performed on twenty-one patients with hyperprolactinaemia without evidence of pituitary tumour and in nine subjects with Turner's syndrome (TS) before and after sulpiride administration (200 mg/day orally, for 3 months). In patients with HA an enhanced response of gonadotrophins to GnRH was observed. In patients with TS a further increase of the gonadotrophin hyper-responsiveness to the releasing hormone occurred when hyperprolactinaemia was induced by chronic sulpiride administration. Since, in these patients, appreciable ovarian steroidogenesis is lacking, a direct effect of the increased plasma PRL at the hypothalamo-pituitary level is likely. In hyperprolactinaemic amenorrhoea pituitary hyper-responsiveness may not be due to PRL effects on ovarian steroidogenesis.