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1. Paired frog sartorius muscles were exposed to Ringer solutions labelled with ²²Na⁺ for about 20 min. At the end of this exposure one of them was stimulated supramaximally one hundred to two hundred times. Immediately after the stimulation both members of the pair were washed in a series of tubes filled with a Na⁺-free medium containing 3 x 10^-5 M strophanthidin.

2. Under the above conditions the intracellular component of the efflux was exponential with an average time constant () of 388 min, that is, approximately four times longer than in the presence of normal Ringer. On the other hand the mean for the washout of the interfibre space was 3·2 min.

3. From the extrapolation to time zero of the intracellular component of the washout curve the initial intracellular radioactivity of both muscles was obtained and the resting and extra Na⁺ influx were calculated.

4. The mean surface membrane area/muscle weight ratio was found to be 552 cm².g^-1 and the mean fibre diameter 53·4 µm for muscles weighing on the average 60 mg.

5. The average resting Na⁺ influx in the presence of normal Ringer was 4·7 p-mole.cm^-2.sec^-1. As the external Na⁺ concentration ([Na⁺]₀) was reduced the Na⁺ influx diminished in a non-linear fashion. This non-linearity could be accounted for by the presence in the influx of a Na⁺ for Na⁺ exchange fraction which saturates at low [Na⁺]₀.

6. The mean extra Na⁺ influx in the presence of normal Ringer was 27·4 p-mole.cm^-2.impulse^-1 and was not significantly affected either by halving [Na⁺]₀ or by varying the frequency of stimulation. When [Na⁺]₀ was reduced to 45 mM by partial replacement of Na⁺ by Tris⁺ the extra influx was significantly higher than when choline⁺ instead of Tris⁺ was used to substitute for Na⁺.

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