We investigated the contribution of extracellular Na+ to the multiple-quantum filtered 23Na NMR signal of perfused rat hearts to determine if the presence of shift reagent Dy(PPPi)2 and inorganic phosphate were somehow responsible for the generation of extracellular multiple quantum coherence. Neither phosphate nor shift reagent caused an increase in the total multiple-quantum filtered signal intensity or in the percent contribution from extracellular ions. On the contrary, addition of Dy(PPPi)2 actually decreased the total signal intensity from intra- and extracellular ions. Further addition of 1.5 mM Gd(PPPi)2 eliminated the extracellular contribution. These data indicate that the previously reported extracellular contribution in perfused hearts is a true contribution of extracellular ions, and not an artifact originating from their interaction with the shift reagent.