The purpose of the study was to evaluate the acute effect of daily subcutaneous injections of the GnRH agonist Buserelin on serum concentration of LH (evaluated by radioimmunoassay (iLH), and by bioassay (bioLH], FSH, oestradiol and testosterone in 20 women with a diagnosis of polycystic ovarian disease (PCO). Buserelin was administered subcutaneously at the dose of 200 micrograms three times daily for 7 days followed by 500 micrograms once daily for a total treatment period of 6 months. Blood samples were drawn immediately before and 2, 4 and 6 h after the injection on the first day of treatment as well as after 1, 2, 3, 4, 12 and 24 weeks. The first injection of Buserelin induced statistically significant rises of iLH, bioLH and FSH as well as of 17 beta-oestradiol. The acute response of gonadotrophins was suppressed after 1 week but a partial recuperation of FSH and iLH responses was observed starting after 2 and 4 weeks of treatment. No significant acute changes of bioLH or of 17 beta-oestradiol and testosterone could be seen. The persistence of a small but statistically significant pituitary response in the first hours following the daily injection of the peptide suggests a hyper-reactive state of the gonadotrophs in PCO patients. Such acute pituitary responses are not seen when a similar treatment is administered to endocrinologically normal women. This observation could be an argument in favour of an initial central defect in the pathogenesis of the syndrome.