CGS-16949A is a new orally active nonsteroidal aromatase inhibitor which is more than 100-fold more potent than aminoglutethimide. This compound is an imidazole derivative, and therefore, its possible effect on cytochrome P-450-dependent enzyme activities in the adrenal gland was evaluated. In vitro investigations with dispersed normal and hyperplastic human adrenocortical cells showed that CGS-16949A at 10(-7)-10(-6) M is a potent 11 beta-hydroxylase inhibitor, which inhibits ACTH-stimulated cortisol release to a similar extent as an equimolar concentration of metyrapone (IC50 for both compounds, 10(-7)-5 X 10(-7) M). Etomidate was a more potent 11 beta-hydroxylase inhibitor (IC50, approximately 10(-8) M), while 10(-7)-10(-6) M ketoconazole caused (via 17 alpha-hydroxylase inhibition) a similar inhibition of cortisol release as 10(-7) M CGS-16949A (IC50, 10(-7)-5 X 10(-7) M). The 11 beta-hydroxylase inhibition by CGS-16949A was accompanied by a dose-dependent increase in the release of precursor steroids by the adrenocortical cells in vitro, including deoxycortisol, 17-hydroxyprogesterone, and androstenedione. Aldosterone release was suppressed 50% by 10(-9) M CGS-16949A, while the IC50 for cortisol in the same cells was 10(-7) M. Aldosterone release by the dispersed adenoma cells obtained from a patient with primary aldosteronism was also significantly suppressed by CGS-16949A. We concluded that 1) the new nonsteroidal aromatase inhibitor CGS 16949A is an inhibitor of 11 beta-hydroxylase which is equipotent to metyrapone. At present it is unclear whether the compound at the dose that causes complete aromatase inhibition in vivo also affects stress-induced cortisol release in man. 2) CGS-16949A exerts a very potent inhibitory effect on normal aldosterone release (IC50, 10(-9) M) and on tumorous aldosterone secretion. CGS-16949A might, therefore, be a drug that can be used in the treatment of primary hyperaldosteronism.