The inhibitory effect of dopamine on aldosterone secretion was investigated in patients with different types of primary aldosteronism, six with idiopathic hyperaldosteronism (IHA) and four with dexamethasone-suppressible hyperaldosteronism (DSH), and in 10 patients with essential hypertension. The effects of 10 mg metoclopramide given intravenously, 10 mg bromocriptine given orally and 100 micrograms adrenocorticotrophic hormone given intravenously on plasma aldosterone and renin activities were investigated in all patients. Metoclopramide induced a rise in plasma aldosterone activity only in patients with IHA and not in those with DSH and essential hypertension. After bromocriptine plasma aldosterone concentrations decreased in patients with IHA only, and after adrenocorticotrophic hormone plasma aldosterone concentrations increased in patients with DSH only. Plasma renin activity was unaffected in all cases. These results provide evidence of increased endogenous dopaminergic inhibition of aldosterone secretion in IHA and of a blunted aldosterone response in both DSH and essential hypertension.