1. A voltage‐clamp technique for the measurement of effective membrane capacity is described and its theoretical basis given in an Appendix. Unlike most other methods used so far, the present technique does not depend on the assumption of a particular equivalent circuit for muscle membrane. 2. With this technique, we have re‐examined the effect of media of low ion content on the effective membrane capacity Ceff. In fibres where potassium conductance is blocked by Rb, Ceff failed to show any appreciable dependence on ionic strength (between 0·015 and 0·29 M) or conductivity (between 0·86 and 13·4 mmho/cm) of the bathing medium. 3. Low conductivity can reduce Ceff in fibres of moderately high K‐conductance. However, in all cases explored here, Ceff had values well above the 2 μF/cm2 found in glycerol‐treated fibres, indicating that passive spread of potential displacements from surface into the tubular system could still occur. 4. The changes in Ceff observed under conditions of moderately high K‐conductance are explicable as a result of potential decrements in the transverse tubules, which would be expected when the wall conductance there is high, and the conductivity of the tubule lumen is low.
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