The present study was conducted to examine potassium lowering effect of exogenous mineralocorticoid (fludrocortisone acetate; FCA) administration to the patients with chronic renal failure undergoing hemodialysis. Fifteen patients on hemodialysis receiving FCA with its dosage gradually increased from 0 to 0.20 mg/day were observed for five successive 4-week periods. The serum potassium concentration was significantly decreased after FCA administration concomitant with the decrease of the salivary sodium to potassium ratio. Such decrease in serum potassium concentration was more significant in patients with <150 pg/ml of plasma aldosterone concentration (PAC) (low PAC group) than in those with ≧150 pg/ml of PAC (high PAC group). 0.05 mg of FCA was sufficient to lower serum potassium in low PAC group, while 0.15 mg of FCA was required for high PAC group. FCA administration did not affect serum sodium, chloride and bicarbonate concentrations. Body weight and blood pressure were not increased during the experimental periods. There were no significant changes in plasma level of glucose, insulin, epinephrine and norepinephrine. These results suggested that FCA could be effective to treat hyperkalemia without any adverse effects in patients undergoing hemodialysis.