Evidence is presented that the histologic lesion and the defect in urinary concentrating capacity seen in potassium deficiency are a consequence of the potassium deficiency per se and their progression is a function of the duration of the potassium deficiency, not of its severity. Prolonged potassium deficiency results in irreversible scarring of the kidney and in an irreparable defect in the urinary concentrating capacity. Dilution of urine in potassium deficiency is minimally impaired at a time when a defect in urinary concentrating capacity is present. A relation between the histologic lesions in the collecting tubules of the potassium deficient animals and the defect in the urinary concentrating capacity of these animals is discussed.