Renal mechanisms for metabolic acidosis in selective hypoaldosteronism were investigated in a patient with marked renal insufficiency. Studies demonstrated a distal hydrogen ion secretory defect. Control of elevated serum potassium levels did not normalize urinary acid excretion, but systemic acidosis was improved. Mineralocorticoid therapy normalized serum potassium without increasing urinary potassium. Hyperkalemia did not directly affect renal acid handling, and extrarenal mechanisms for potassium excretion are suggested. Measurement of rectal mucosal potential difference suggests this test may be of value in detecting a wide range of abnormalities in aldosterone metabolism.