Nondestructive observation of intracellular sodium (Na+i) levels is of utmost clinical and biochemical importance, 23Na is a relatively sensitive nuclear magnetic resonance (NMR)-observable nuclide, and NMR is intrinsically noninvasive. Since, however, the frequency position of Nai+ and Nao+ is identical, paramagnetic dysprosium complexes have to be used as shift reagents. The latter differentiate between signals from intracellular and extracellular spaces, providing a nondestructive, continuous method to monitor intracellular cation concentration, in real time. In this article, the methodology of shift-reagent-aided 23Na NMR spectroscopy is discussed. Application is demonstrated by results from two studies in isolated perfused rat hearts. In hearts subjected to hypoxia (or ischemia) followed by reoxygenation (or reflow), the authors show that recovery following an ischemic or hypoxic insult can be predicted by monitoring Na+i levels by NMR. In a study in paced hearts, the authors show an accumulation of Na+i with increased heart rate, and also a positive coupling between elevated sodium levels and increased systolic pressure during the pacing-up period, as predicted by the Na-pumplag theory.