Time-related changes in plasma levels of aldosterone, deoxycorticosterone, corticosterone, and cortisol were studied during treatment with spironolactone in 8 patients with primary aldosteronism due to adenomas. Plasma renin activity (PRA), serum sodium and serum potassium were also measured. The patients were treated with spironolactone, 75 to 225 mg daily, and blood samples were withdrawn on days 7, 14, and 21 to 28 of drug administration. Plasma aldosterone concentrations (PAC) were not altered by spironolactone; however, significant increases were observed in plasma deoxycorticosterone on days 21 to 28 and in plasma corticosterone and cortisol on days 14 and 21 to 28. The suppressed PRA values were markedly increased on days 21 to 28 and low levels of serum potassium returned to the normal range on day 7. The lack of increase in PAC, despite a remarkable rise in PRA and serum potassium, suggests biosynthetic inhibition of spironolactone at the sites of 18-hydroxylation and/or 18-oxidation, because of the elevation of deoxycorticosterone and corticosterone.