In contrast with other dopaminergic drugs (L-dopa, apomorphine or bromocriptine) a 280 mug/min dose of dopamine infused for 120 min failed to induce an increase in plasma GH in 9 normal subjects. During dopamine infusion, no significant change in the GH response to arginine was also noted, whereas prolactin displayed a significant fall. In 15 acromegalic patients, on the other hand, the drug caused a marked fall in both GH (mean+/-SE; 71.1%+/-5.6) and prolactin (mean+/-SE; 67.6%+/-4.0) followed by a distinct rebound after the end of the test. There was a very close relation (P less than 0.001) between the maximum per cent decrease in GH during dopamine and after 2.5 mg bromocriptine by mouth. Dopamine also inhibited the GH response to TRH (4 patients). Since dopamine does not readily cross the blood-brain barrier, these results suggest that the stimulating effect of dopaminergic drugs on GH secretion in the normal subject is exerted via the CNS, whereas in acromegaly there is a direct action on structures lying outside the blood-brain barrier, probably in the hypophysis. Dopaminergic inhibition of prolactin is primarily the result of action on the hypophysis, as well as on the hypothalamus, in both normal and acromegalic subjects.