Nomifensine, TRH and insulin-induced hypoglycemia tests were carried out in 37 cases of hyperprolactinemia: 25 were due to PRL-secreting pituitary tumors, 6 cases to GH and PRL-secreting pituitary tumors and 6 to pituitary and suprasellar non secreting tumors. Nomifensine failed to suppress the serum PRL in all subjects and PRL responses to TRH and insulin-induced hypoglycemia were impaired in all patients, irrespective of the origin of hyperprolactinemia. The uniform pattern of PRL response to the above tests in patients with hyperprolactinemia of variable etiology suggests that none of them is specific for prolactinomas.