The cation content of the erythrocyte was studied in 36 patients with chronic renal failure. A normal or high erythrocyte K was found. Erythrocyte Na values showed a wide range, from very high to very low and the rate constant for Na efflux was higher than normal. The interpretation of these findings was discussed. The high erythrocyte K content may reflect abnormal total body K, may be explained on the basis of a young red cell population or may be related to abnormal function of the erythrocyte cation pump. The development of these changes in chronic renal failure probably depends on many factors.