Although aldosterone (Aldo.) secretion is regulated by various humoral factors, evidence has accumulated to support an involvement of dopaminergic system in its regulation. The pathophysiological significance of the dopaminergic system in primary aldosteronism (PA) however remains unknown. In the present study, we examined the effects of metoclopramide (MCP) on Aldo. secretion in normal subjects (n=11) and patients with essential hypertension (EH, n=8), aldosterone-producing adenoma (APA, n=10), and idiopathic hyperaldosteronism (IHA, n=6). Plasma Aldo., prolactin (PRL), renin, cortisol, serum sodium, and serum potassium levels were determined before and 30min after i.v. bolus injection of 10mg MCP at 9 a.m. Plasma Aldo. showed a significant increase after MCP in normal subjects, EH, and APA, but not in IHA. The incremental response of plasma Aldo. was largest in APA and smallest in IHA. The percentage increase in plasma Aldo. from the basal level was significantly attenuated in IHA, while no significant difference was seen among other groups. Although plasma PRL showed a significant increase in response to MCP, no difference of the change was seen among the groups. There was no significant change in plasma cortisol, renin, serum sodium, and serum potassium levels in response to MCP. In addition, the response of Aldo. to MCP was normalized in APA after unilateral adrenalectomy, while that of PRL did not change. These results indicate that the adrenal dopaminergic activity is enhanced in APA and attenuated in IHA and suggest an involvement of the dopaminergic system in the pathogenesis of IHA. The MCP challenge test could be an useful diagnostic tool for the differential diagnosis of subsets of PA. (Hypertens Res 1995; 18 Suppl. I: S59-S64)