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1. The intracellular Na activity of sheep heart Purkinje fibres has been measured using recessed-tip Na⁺-sensitive glass micro-electrodes.

2. The internal Na activity was 7·2 ± 2·0 mM (mean ± S.D., n = 32) at the normal external Na concentration, [Na]_o, in these experiments of 140 mM (equivalent to an external Na activity of 105 mM). The equilibrium potential for Na across the fibre membrane was therefore approximately + 70 mV.

3. When the [K]_o was altered the internal Na activity changed, reaching a new level within about 20 min. Increasing the [K]_o from 4 to 25 mM decreased the internal Na by approximately 30%, while decreasing the [K]_o from 4 to 1 mM increased internal Na by 20%.

4. The removal of external K produced an easily reversible increase in the internal Na with an initial rate equivalent to a concentration change of 0·24 ± 0·07 m-mole/min (mean ± S.D., n = 8).

5. Ouabain produced increases in the internal Na activity that were only very slowly reversible. The threshold concentration for producing an increase was approximately 10^-7 M.

6. When [Na]_o was reduced the internal Na activity fell rapidly with a single exponential time course (time constant 3·3 ± 0·8 min, mean ± S.D., n = 16) to a new, relatively stable level. The recovery of internal Na on return to the normal [Na]_o did not have a simple time course. It was normally complete within 10-30 min.

7. The relationship of the stabilized level of the internal Na activity to the [Na]_o was approximately linear over the range 140-14 mM-[Na]_o. When [Na]_o was reduced from 140 to 14 mM the internal Na activity fell by 72 ± 5% (mean ± S.D., n = 21).

8. When the [Na]_o was reduced, the decrease in the internal Na activity was partially inhibited by Mn or by removal external Ca.

9. When the [Ca]_o was altered over the range 0·2-16 mM the internal Na activity was reduced by approximately 50% for a tenfold increase in the [Ca]_o.

10. The relationship between internal Na and contractility is discussed.

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