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1. The overshoot of the action potential of the frog's heart was reduced when external sodium chloride was replaced by sucrose. However, the potential decrement was only 17·3 mV for a 10-fold reduction of sodium as compared with 58 mV expected on the basis of the sodium hypothesis of excitation.

2. Replacement of up to 75% of the external sodium by choline did not reduce the overshoot, provided atropine was present in sufficient concentrations to suppress any parasympathomimetic action.

3. The maximum rate of rise of the action potential markedly declined in low sodium fluids whether sucrose or choline chloride was used to replace sodium chloride.

4. The maximum rate of rise was reduced to only a small extent when external sodium was replaced by lithium.

5. Increasing the intracellular sodium concentration in exchange for lost potassium caused overshoots to decline. The effects resembled those obtained in similar experiments with skeletal muscle fibres (Desmedt, 1953).

6. Action potentials occurring under certain conditions even in the presence of very low external sodium concentrations ( 5% normal) also declined in height when the intracellular sodium concentration was increased.

7. The behaviour of the action potential in low external sodium concentrations may be explained by an action of calcium on the excitable membrane.

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