Domperidone, a peripheral dopamine (DA) receptor blocker which poorly crosses the blood-brain barrier and which is inactive towards dopamine-sensitive adenylate cyclase, in a dose (100 micrograms/kg) sufficient to increase serum prolactin levels at least 5-fold, decreased the growth hormone (GH) response to the DA receptor agonist, apomorphine HCI (Apo) (0.5 gm s.c.) in each of six normal men examined. The mean GH increment at 30, 45, 60 and 75 min following Apo injection, the mean individual peak increment and the mean individual GH secretion (ng min) was significantly decreased by domperidone pretreatment (p less than 0.005 -p less than 0.002). These results indicate that in man Apo stimulates GH secretion by an effect on DA receptors which are not linked to adenylate cyclase and which are situated at a locus in the hypothalamic-pituitary axis that lies outside the blood-brain barrier.