Basal serum aldosterone levels in 13 hyperprolactinaemic females did not differ significantly from those of nine control individuals. There was increased responsiveness of circulating aldosterone levels to the long acting dopamine antagonist metoclopramide (10 mg intravenously) in the hyperprolactinaemic patients as compared with the controls. Prolactin responsiveness to metoclopramide was reduced in the patients as compared with the controls, such being considered characteristic of a prolactinoma. Basal serum thyrotrophin (TSH) levels, although within the euthyroid range, were increased in the patients as compared with the controls. There was no significant difference in the TSH responsiveness to metoclopramide between the study groups. However, in the five patients with exaggerated responses of TSH to metoclopramide, basal TSH levels were significantly higher than in the other patients. One explanation for these results is that prolactin can directly or indirectly modulate the aldosterone response to metoclopramide.