Metoclopramide, a dopamine receptor antagonist, increases plasma aldosterone concentration in man, suggesting that dopamine regulates the secretion of aldosterone. In the current study, we administered metoclopramide to rhesus monkeys and normal subjects and compared the time-course and dose-response characteristics of plasma aldosterone. We also examined the effect of dopamine on the plasma aldosterone response to metoclopramide in both species. Six male rhesus monkeys and seven normal subjects (five women and two men) were studied on diets providing an estimated daily sodium intake of 70 mg⁄kg. In both species the peak increase in plasma aldosterone occurred 15 min after metoclopramide was injected. The peak plasma aldosterone value was 3-fold higher than control values. There were no significant changes in PRA, cortisol or potassium, whereas plasma PRL increased 7-fold in the monkeys and 11-fold in the normal subjects. After 0.04 mg⁄kg metoclopramide, there was no change in plasma aldosterone concentration in the monkeys, whereas aldosterone increased significantly (A = 3.7 ± 0.68 ng⁄dl) in the human subjects. The half-maximal dose of metoclopramide was also higher in the monkeys than in the normal subjects. A dopamine infusion at 4.0 to 8.0 jug⁄kg · min partially suppressed the plasma aldosterone response to metoclopramide in both the human subjects and the monkeys. This study demonstrates that metoclopramide produces doserelated increases in plasma aldosterone concentration in the nonhuman primate that are similar to those in normal man and that the increases can be inhibited by dopamine. We conclude that aldosterone secretion may be under dopamine control and that therhesus monkey should be an excellent model in which to study further the regulation of aldosterone by dopamine.