Active transport of Na and K under physiologic and maximally stressed conditions was identical innormaladult red blood cells (RBC) and term neonatal erythrocytes. These results are consistent with the previous observation that Na-K ATPase is the same innormaladult RBC and term neonatal erythrocytes. These data, however, are at variance with a previous observation that active K transport is impaired in neonatal erythrocytes. The most reasonable explanation for this difference relates to inherent problems with the use of radioisotopes which were used in previousin vitrostudies of cation transport.