The use of thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate (TmDOTP5-) as an in vivo 23Na NMR shift reagent for rat liver was evaluated by collecting interleaved 23Na and 31P spectra. Infusion of 80 mM TmDOTP5- without added Ca2+ produced baseline-resolved peaks from intra- and extracellular sodium without producing any changes in phosphate metabolite resonances or intracellular pH. Several key physiological parameters measured in parallel groups of animals confirmed that liver physiology is largely unaffected by this shift reagent. A direct comparison of TmDOTP5- versus DyTTHA3- showed that after infusion of 5-8 times more DyTTHA3-, the extracellular sodium peak shifted by the same amount as with TmDOTP5-, but the two 23Na resonances were very broad and not resolved. The baseline-resolved peaks with TmDOTP5- allowed us to measure the in vivo T1 and T2 relaxation characteristics of intra- and extracellular Na+. The measured T1, T2s, and T2f values and the relative contributions from the slow and fast T2 components for intracellular Na+ in liver did not differ significantly from the values reported for perfused frog heart. The T1 and T2 relaxation curves of the extracellular Na+ resonances fit a monoexponential function. Analysis of the relative contribution of the fast- and slow-relaxing T2 components from intracellular Na+ resulted in a calculated visibility factor of 69 +/- 4% and the intracellular Na+ concentration calculated from the NMR peak intensity ratio, the measured visibility factor, and literature values of intra- and extracellular volume was 19 mM. These results indicate that TmDOTP5- promises to be quite useful as an in vivo shift reagent for liver and other organs.