Magnesium (Mg) deficiency is a common clinical problem. As Mg is predominantly an intracellular cation and Mg deficiency may exist despite normal serum Mg (sMg) concentrations, we have utilized nuclear magnetic resonance (NMR) techniques in an attempt to measure intracellular free Mg (Mg2+) in red blood cells (RBC). Twenty normal subjects, 22 hypomagnesemic patients, and 17 normomagnesemic alcoholic patients were studied. Mean RBC Mg2+ in normal subjects (178 +/−6.3 microM) was significantly greater than in hypomagnesemic patients (146 +/−7.1 microM, p less than 0.002). RBC Mg2+ correlated with sMg concentration (r = 0.54, p less than 0.001). In addition, four normal subjects were given a low Mg diet for 3 weeks. There was a progressive fall in both the sMg concentration and RBC Mg2+ during Mg depletion, with a concomitant rise in retention of a parenterally administered Mg load. These data suggest that the determination of intracellular Mg2+ by NMR may be a useful research tool in assessing the effect of changes in Mg2+ on intracellular processes. Its utility in the clinical evaluation of disorders of Mg deficiency remains to be determined.