Ten normoprolactinemic and 10 hyperprolactinemic patients, all with polycystic ovary syndrome (PCO), were subjected to prolactin (PRL) stimulatory tests with thyrotropin-releasing hormone (TRH), 200 microgram intravenously, and haloperidol (a dopamine-blocking agent), 1 mg intramuscularly. The results were compared with those of 8 women with idiopathic hyperprolactinemia and 10 normal female volunteers. Distinctive features of PCO were elevated plasma concentrations of luteinizing hormone, estrone, and testosterone in the presence of normal estradiol, whereas in idiopathic hyperprolactinemia estradiol was reduced. Both groups of patients with PCO exhibited responses to TRH and haloperidol significantly higher than the controls (P less than .001), whereas only the hyperprolactinemic PCO patients reacted with an excessive PRL discharge (P less than .001). As expected, the response to both secretagogue agents was blunted in patients with idiopathic hyperprolactinemia. The present report discusses the possible implication of estrogen and the dopaminergic system in the mechanisms leading to hyperprolactinemia and enhanced PRL release in PCO.