The serum TSH and prolactin responses to dopamine receptor blockade (domperidone 10 mg i.v.) were measured in eight acromegalic subjects and seven normal controls. The serum TSH response was exaggerated in acromegalics when compared to controls (sum of the increments 12.2 +/- 2.8 mU/l and 4.0 +/- 1.2 mU/l respectively P less than 0.05), whereas the serum prolactin response was blunted (sum of the increments 5029 +/- 1377 mU/l and 14 673 +/- 2889 mU/l respectively P less than 0.01). Dopamine receptor blockade did not significantly alter basal growth hormone levels in either group. To ascertain whether this response was merely a reflection of high circulating growth hormone levels six normal controls were pretreated for 24 h with either intramuscular growth hormone (three injections of 4 IU) or an equivalent volume of saline before the administration of domperidone (10 mg i.v.). The serum TSH and prolactin responses were not affected by prior growth hormone treatment. The opposite TSH and prolactin responses to dopamine blockade do not allow any firm conclusions to be made about the level of hypothalamic dopamine activity in acromegaly.