In order to evaluate whether hyperprolactinaemic patients have an increased or decreased central dopaminergic tonus, we compared the serum Prl, TSH, GH and LH responses to metoclopramide (10 mg iv) and to breast-feeding in normal women (n = 8) and hyperprolactinemic patients (n = 8). Prl increments to metoclopramide were significantly (P less than 0.05) impaired in hyperprolactinaemic patients. Serum TSH increased significantly after metoclopramide both in normal and hyperprolactinaemic patients (P less than 0.025), but no significant differences were found between the two groups regarding TSH, GH or LH levels. Hyperprolactinaemic patients had no acute response to suckling while significant increments of Prl and GH (P less than 0.025) occurred in normal women. Out data indicate that the central dopaminergic tonus was not altered in hyperprolactinaemic patients, but showed regulatory dysfunction of both Prl and GH. The concept of a dopaminergic inhibition of TSH secretion was confirmed.