We have used a combination of a shift reagent and mathematical filtering or presaturation of the extracellular sodium resonance for the quantitative investigation of the intracellular sodium and lithium relaxation times in the perfused frog heart. While the T1 of the intracellular sodium was found to consist of a single-exponential time constant (approximately 23 ms), the T2 was better fit as a double-exponential decay with time constants of approximately 2 and 17 ms. However, the relative amplitudes of the two time constants in the T2 decay were found to be inconsistent with those which would be expected from a homogeneous pool of nuclei undergoing quadrupolar interactions. The relaxation times were not changed by a fivefold increase in the intracellular sodium level (due to perfusion with a ouabain-containing buffer). The T1 and T2 of the intracellular lithium (after perfusion with lithium-containing buffer) were both well fit by single exponentials (700- and 31-ms time constants, respectively).