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1. The basis for the relatively high membrane capacitance of the cardiac Purkinje fibre has been investigated.

2. The capacitance measured by analysis of the cable response to a current step (square wave) was compared in the same fibres to the capacitance calculated from the foot of the propagated action potential. The square wave value for capacitance was 12·8 ± 1·3 µF/cm² and that from the foot of the action potential, 2·4 ± 0·5 µF/cm².

3. The capacitative filling at the beginning of a voltage clamp in short Purkinje fibres was measured. The current-time course deviated from that predicted by a model membrane containing resistance and capacitance in parallel.

4. The results obtained by both methods are consistent with two components to the membrane capacitance, with part in parallel with the membrane resistance (2·4 µF/cm²) and part (7 µF/cm²) in series with a resistor (300 cm²).

5. The value of the series resistor could be increased by decreasing the conductivity of the extracellular fluid.

6. The possible anatomical basis for these findings is discussed.

7. Implications of this model on the shape of the Purkinje fibre action potential and on the electrical triggering of contraction are considered.

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