This study was designed to investigate the role of dopaminergic mechanisms in the control of aldosterone secretion. Six rhesus monkeys received metoclopramide (1.25 mg, iv), with 5% dextrose (vehicle) or with dopamine (4 micrograms/kg . min) infusions begun 60 min before the administration of the dopamine antagonists. Metoclopramide, in the presence of vehicle, increased plasma aldosterone concentrations from 4.5 +/- 0.5 ng/dl to a maximum of 26 +/- 4.1 ng/dl and PRL concentrations from 8.1 +/- 1.3 ng/ml to a maximum of 118.4 +/- 7.6 ng/ml. Dopamine infusion inhibited the aldosterone and PRL responses to metoclopramide. The administration of metoclopramide resulted in a rise in plasma 18-hydroxycorticosterone from 10.4 +/- 1.5 ng/dl to a maximum concentration of 41 +/- 4.2 ng/dl. The aldosterone and 18-hydroxycorticosterone responses displayed a parallel time course, with significant responses of both occurring 5 min after metoclopramide administration. Plasma concentrations of electrolytes, PRA, plasma cortisol, 11-deoxycorticosterone, corticosterone, and 18-hydroxy-11-deoxycorticosterone were not altered by metoclopramide. The results of this investigation demonstrate that aldosterone and PRL responses to metoclopramide are mediated by their antagonist activities at dopamine receptors. Rather than simply affecting secretion, dopaminergic mechanisms appear to modulate the late pathway of adrenal glomerulosa biosynthesis of aldosterone. A parallel time course of stimulation of 18-hydroxycorticosterone and aldosterone secretion, with no change in other aldosterone precursors, strongly suggests that dopamine modulates the activity of the glomerulosa 18-hydroxylase enzyme.