Triethylenetetraminehexaacetate complexes of Dy(II) or Tm(III) (DyTTHA3- and TmTTHA3-, introduced as NMR shift reagents for alkali metal cations, Chu et al., J. Magn. Reson. 56, 33 (1984) bind to the four major biological inorganic cations: Na+, K+, Mg2+, and Ca2+. New 23Na and 39K NMR shift displacement data, obtained over wide and different ranges of concentration, were combined with the previous results (including some 25Mg data) for computer analysis. With a proper treatment of the relevant activity coefficients using Pitzer's formulas, it is established that only mono complexes of the shift reagent and a cation need be considered. The cations bind competitively, with nearly identical limiting shifts of 159 and 155 ppm for Na+ and K+; the shift for 25Mg is 3 5 as large. The thermodynamic formation constants for M-LnTTHA are 11 and 18 M-1 with Na+ and K+ (enthalpy of binding, -54 kJ), and 130 and 3100 M-1 with Mg2+ and Ca2+ at 3°C. The results suggest that all the cations bind at the same site, with Mg2+ probably forming a solvent-separated complex. The formation constants expected at 37°C indicate that, at useful shift reagent concentrations, and at physiological cation concentrations, DyTTHA3- can be used not only to distinguish among cation pools but also for quantitative studies of cation relationships. In particular, 23Na signals might be used to report free extracellular Ca2+ concentrations in vivo.