1. Using (24)Na to label the exchangeable sodium in the tissue and either [(35)S]sulphate or [(14)C]sucrose to label the extracellular spaces, the intracellular sodium concentration of frog heart ventricles was determined and found to be between about 5 and 10 m-mole/kg cell water.2. The intracellular potassium concentration, obtained by flame-photometric analysis, was approximately 163 m-mole/kg cell water.3. Two different methods were employed to study the sodium tracer efflux of resting heart ventricles. One involved a double-tracer technique, using (24)Na to indicate the release of the exchangeable tissue sodium, and (35)SO(4) to indicate, approximately, the release of extra-myocardial sodium. In the other a comparison was made of the sodium release from the tissue when it contained sodium either at the normal concentration or at a concentration enhanced by exchange with intracellular potassium.4. The magnitude of the sodium efflux from heart fibres as measured by both methods was of the order 50-100 p-mole/cm(2) sec. Simultaneously with this cellular efflux a substantial amount of sodium was released from extra-myocardial spaces and tissues.5. The net efflux of potassium ions from heart cells that occurred when heart ventricles were perfused with potassium-depleted fluids was determined. The results were used to obtain an indirect estimate, of about 2-3 p-mole/cm(2) sec, for the resting sodium influx.6. The significance of these different values of sodium efflux and influx is discussed.