Six patients with primary aldosteronism (PA), one with idiopathic hyperaldosteronism (IHA), one with glucocorticoid responsible hyperaldosteronism (GRHA) and eight with essential hypertension (EH) were treated with trilostane (MWD-1822) (4 alpha, 5-epoxy-17 beta-hydroxy-3-oxo-5 alpha-androstane-2 alpha-carbonitrile), an inhibitor of adrenal steroid biosynthesis, for 9-47 days with a daily dose of 30-960 mg. Blood pressure decreased slightly and gradually from 30 min. to 360 min, plasma aldosterone (PAC) and cortisol concentration (F) decreased, and plasma dehydroepiandrosterone concentration (DHEA) increased 120 min. after the administration of a single dose of 120 mg of trilostane. In the patients with PA, IHA and GRHA on long term therapy with trilostane, blood pressure decreased, PAC and F were depleted, serum improved within normal limits and DHEA increased, but plasma progesterone concentration (Prog.) changed variously and plasma renin activity (PRA) remained suppressed. In the patients with EH, systolic pressure decreased in 5 out of 8 (under - 20 mmHg), and diastolic pressure decreased in 3 out of 8 (under - 10 mmHg), DHEA increased in all, but the changes in serum potassium, PAC, F, Prog. and PRA were various. There was no remarkable reaction after the administration of trilostane. It is concluded that trilostane is an effective inhibitor of 3-hydroxysteroid dehydrogenase in vivo and that it is useful in the treatment of primary aldosteronism and other hypertension due to hyperproduction of aldosterone.