Abstract: New methods for rapid, noninvasive determination of sodium ion in vivo are described. The methods are useful for assessing liver and other organ injury based on a nuclear magnetic resonance determination of intracellular and extracellular sodium in the damaged tissue. A shift reagent, TmDOTP.sup.5-, is used in vivo to produce baseline resolved peaks of .sup.23 Na. Excellent results are achieved with significantly less TmDOTP.sup.5- than with other conventional .sup.23 Na shift reagents such as DyTTHA.sup.3-. In contrast to use in isolated perfused heart tissue, TmDOTP.sup.5- solutions are employed in vivo without added calcium ion. The method has been applied to animal burn models. Results show that intracellular sodium ion levels are dramatically increased, thus providing a rapid assessment of liver function. Similar methods may be applied to the determination of intra- and extracellular potassium ion.