Na<sup>+</sup>, K<sup>+</sup>-ATPase activity in erythrocytes (RBC) has been assessed indirectly by the <sup>86</sup>Rb uptake method in 10 patients with acute renal failure (ARF), 11 patients with chronic renal failure (CRF) on haemodialysis and 13patients treated by kidney transplantation, and compared to age-matched healthy controls. In each patients, the sensitivity of <sup>86</sup>Rb uptake to digoxin (IC<sub>50</sub>) has been measured. Na<sup>+</sup>, K<sup>+</sup>-ATPase activity was decreased in patients with ARF and in patients with CRF but not in transplanted patients. IC<sub>50</sub> was lowered in transplanted patients. The implications of these findings are discussed in terms of possible mechanisms of the changes in Na<sup>+</sup>, K<sup>+</sup>-ATPase activity in RBC of patients with renal failure.