Effects of spironolactone, canrenone and canrenoate-K on adrenal cytochrome P450 (P450) and corticosteroid biosynthesis were examined by studying difference spectra, P450 reduction and corticoid hydroxylation in mitochondrial preparations isolated from zona fasciculata and zona glomerulosa of bovine adrenals and from adrenal adenoma and hyperplastic adrenal cortex removed from patients with hyperaldosteronism. All three agents bound to P450 producing type I difference spectra and underwent hydroxylation. They all inhibited 11beta-hydroxylation in bovine adrenal at 30 muM and higher concentrations. Canrenone, the most potent inhibitor, blocked enzyme activity by 60% at a concentration of 60 muM. Spironolactone stimulated P450 reduction. The order of potency of inhibition was found to correlate with the order of affinity of these agents for P450. 11beta-Hydroxylase in human adrenal appeared to be less sensitive to canrenone. All three agents or their hydroxylated metabolites blocked 18-hydroxylation in bovine adrenal at lower concentrations. Canrenoate-K, being the most effective, inhibited 52% at 20 muM. Low concentrations of canrenone (2.5-5.0 muM) were without effect on 11beta-hydroxylase but markedly inhibited 18-hydroxylation (62-76%) in hyperplastic human adrenals. The inhibitors produced mixed type inhibition of 11beta-hydroxylation and competitive type inhibition of 18-hydroxylation. These findings indicate that at low concentrations spironolactone and its major metabolites, canrenone and canrenoate-K, or their hydroxylated metabolites, can directly interfere with the biosynthesis of aldosterone in bovine and certain human adrenal cortical tissue.