4. Changes in external K concentration produced changes in the initially recorded resting potentials which follow the constant field theory using a ratio of Na:K permeabilities PNa/PK = 0-02. Changes in external C1 concentration produced little or no change in the resting potential or membrane resistance, indicating a low Cl permeability.5. In agreement with previous work, slow fibres showed a timedependent decrease in resistance ('delayed rectification') for membrane potentials more positive than -60 mV. 'Anomalous rectification' observed in twitch fibres was not seen in slow fibres. In high external K concentrations the resistance of slow fibres is almost unaffected by changes in membrane potential.6. Increasing the concentration of external Ca (up to isotonic) has two distinct effects on slow fibres. It increases Rm up to ten times, and it improves the stability of trans-membrane recordings, probably by reducing the leakage due to micropipette penetrations. Magnesium does not appear to have either of these effects.
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