Background— Because of rapid changes in myocardial intracellular Na ⁺ (Na ⁺ _i ) during ischemia and reperfusion (R), ²³ Na magnetic resonance imaging (MRI) appears to be an ideal diagnostic modality for early detection of myocardial ischemia and viability. So far, cardiac ²³ Na MRI data are limited and mostly concerned with imaging of total Na ⁺ . For proper interpretation, imaging of both Na ⁺ _i and extracellular Na ⁺ is essential. In this study, we tested whether Na ⁺ _i imaging can be used to assess viability after low-flow (LF) ischemia. Methods and Results— Isolated rat hearts were subjected to LF (1%, 2%, or 3% of control coronary flow) and R. A shift reagent was used to separate Na ⁺ _i and extracellular Na ⁺ resonances. Acquisition-weighted ²³ Na chemical shift imaging (CSI) was alternated with ²³ Na MR spectroscopy. Already during control perfusion, Na ⁺ _i could be clearly seen on the images. Na ⁺ _i image intensity increased with increasing severity of ischemia. During R, Na ⁺ _i image intensity remained highest in 1% LF hearts. Not only did we find very good correlations between Na ⁺ _i image intensity at end-R and end-diastolic pressure ( R =0.85, P <0.001) and recovery of the rate-pressure product ( R =−0.88, P <0.001) at end-R, but most interestingly, also Na ⁺ _i image intensity at end-LF was well correlated with end-diastolic pressure ( R =0.78, P <0.01) and with recovery of the rate-pressure product ( R =−0.81, P <0.01) at end-R. Furthermore, Na ⁺ _i image intensity at end-LF was well correlated with creatine kinase release during R ( R =0.79, P <0.05) as well as with infarct size ( R =0.77, P <0.05). Conclusions— These data indicate that ²³ Na CSI is a promising tool for the assessment of myocardial viability.