Urinary citrate output was studied in 5 cases of primary aldosteronism, in 3 of Addison's disease and in 8 of normal subjects. The urinary citrate output was decreased in 4 of 5 cases of primary aldosteronism, and the citrate output could not be normalized by removal of adrenocortical adenoma. In patients with primary aldosteronism citrate excretion was somewhat increased by admini-stration of potassium chloride, but rather decreased by spironolactone. In these patients urinary citrate output could not be correlated with either plasma CO2 content or plasma potassium concentration. The low urinary citrate excretion in primary aldosteronism may not be satisfactorily explained either by direct action of aldosterone on the kidney or by intracellular acidosis.