The effect of human atrial natriuretic peptide (hANP) on aldosterone, 18-OH-cortisterone, corticosterone, deoxycorticosterone, and cortisol secretion was examined in isolated human aldosteronoma cells from five patients with primary aldosteronism. hANP exerted a nearly identical inhibitory action on basal secretion of each of these steroids and antagonized also the stimulating effects of ACTH, serotonin, metoclopramide and vasoactive intestinal polypeptide. The results suggest that in human aldosteronoma cells hANP may be a non-selective inhibitor of corticosteroid biosynthesis and that the site of inhibition of steroidogenesis may be localized to the early pathway of steroid biosynthesis.