The action of 9 alpha-fluorohydrocortisone (fludrocortisone) as a potent mineralocorticoid was reviewed. Fludrocortisone shows mineralocorticoid activity as potent as aldosterone and causes sodium-retention and kaliuresis when injected intravenously in man. The duration of the action of fludrocortisone in longer than that of aldosterone. Therefore, fludrocortisone is more pertinent to clinical use than aldosterone. Fludrocortisone is administered as a mineralocorticoid in case of Addison's disease and postoperative Cushing syndrome. It is also used to relieve hyperkalemia or to ameliorate hyponatremia.