Patients with prolactin-secreting pituitary tumours have a diminished prolactin (PRL) response after administration of a variety of stimulatory agents, including thyrotrophin-releasing hormone (TRH), chlorpromazine (CPZ) and insulin-induced hypoglycaemia. We examined responses to these agents in sixty-seven women with PRL-secreting tumours before and after trans-sphenoidal surgery. Twenty-nine of the women were cured as defined by restoration of normal serum PRL concentrations and resumption of regular menses. One year following trans-sphenoidal surgery, patients with normal PRL concentrations and regular menses had normal PRL responses to TRH, while the responses to CPZ and insulin hypoglycaemia were not improved. The reversible impairment of PRL response to TRH reflects suppressed function of normal PRL-secreting cells, and the persistence of abnormal PRL responses to provocative stimuli after successful tumour removal may reflect some fundamental defect in the regulation of PRL secretion in patients with PRL-secreting adenomas. When growth hormone (GH) reserve after insulin hypoglycaemia was represented as the mean absolute increment above the basal concentration and compared with normal subjects, women with PRL-secreting tumours had a diminished response that returned to normal after successful surgery. This reversible impairment of GH secretion is not adequately reflected by standard criteria for GH response and suggests that PRL may have a role in the regulation of GH secretion.