1,4,7-Triazacyclononane-N,N',N''-tris(methylenephosphonate monoethylester) (NOTPME) has been synthesized, characterized and analyzed for use as a 31P NMR indicator of intracellular Mg2+ and Zn2+ ions. The 31P NMR spectrum of this chelate in the presence of metal ions shows characteristic resonances for the free chelate, Mg(NOTPME)-, Zn(NOTPME)-, and Ca(NOTPME)-. The Kd values indicate that this chelate has a 10-fold higher affinity for Mg2+ than for Ca2+ at physiological pH values. In the presence of Mg2+, NOTPME is readily loaded into red blood cells. A 31P NMR spectrum of red cells taken after several washings shows resonances characteristic of entrapped NOTPME and the Mg(NOTPME)- complex, the relative areas of which report an intracellular free Mg2+ concentration of 0.32 mM. The 31P chemical shifts of the free chelate and its metal complexes are far downfield from the typical phosphorus-containing metabolites observed in biological systems, thus making it possible to monitor intracellular cation concentrations and cell energetics simultaneously.