Metoclopramide, a dopaminergic antagonist, has consistently elevated plasma aldosterone levels in vivo. To determine whether this was a direct action of metoclopramide on adrenal steroidogenesis, we examined the response of collagenase-dispersed rat adrenal glomerulosa cells to metoclopramide in vitro. The effect of increasing concentrations of metoclopramide (3 X 10(-10) to 3 X 10(-4) M) on basal as well as angiotensin II (2.4 X 10(-10) to 2.4 X 10(-8) M)-, ACTH (3.5 X 10(-11) M)- and potassium (5.9 meq/liter)-stimulated aldosterone production was evaluated. Metoclopramide caused a dose-related decrease in basal and stimulated aldosterone production (P less than 0.01). In addition, metoclopramide also blocked basal and stimulated corticosterone production (P less than 0.01). This was not due to an irreversible toxic effect, since glomerulosa cells preincubated with 3 X 10(-4) M metoclopramide excluded trypan blue dye and responded to ACTH stimulation. Sodium metabisulfite, an antioxidant present in the metoclopramide preparation, did not contribute to the metoclopramide effect. These results indicate that metoclopramide is an aldosterone antagonist in vitro, contrary to reported data obtained in vivo. Thus, metoclopramide may be a partial dopaminergic agonist: in vitro where dopamine levels are negligible, it is an agonist, whereas in vivo where dopamine concentrations are greater, it is an antagonist.