Aqueous shift reagents were used to clearly distinguish intra-and extracellular 23Na-nuclear magnetic resonance (NMR) signals in samples consisting of whole blood or suspensions of washed human erythrocytes (both fresh and outdated). The lanthanide chelates Dy(PPP)2(7-) and Tm( TTHA )3- were used to shift the extracellular signals upfield, and Dy( TTHA )3- and Tm(PPP)2(7-) were similarly used to shift extracellular resonances downfield. The absolute intensities of the signals were used along with the measured hematocrit to simultaneously determine the intra- and extracellular Na+ concentrations. The results were generally within 5% of the values determined by more time-consuming centrifugation-flame emission photometry measurements on the same samples. Thus the 23Na-NMR signals from both intra- and extracellular cations suffer no NMR invisibility within experimental error. The lower level of intracellular Na+ in fresh erythrocytes (less than 12 mM) is easily distinguished from the higher level (approximately 30 mM) in erythrocytes that have been stored (in the cold) outside the body for some weeks.