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1. The membrane capacitance and slope conductance of sheep cardiac Purkinje tissue were examined under conditions of constant and slowly changing voltage within the potential range: -70 to -100 mV. Data were secured by applying constant current pulses, or by imposing ramp commands or small changes in holding potential under voltage clamp control.

2. The electrical measurements, combined with light and electron microscopic data on preparation structure, were interpreted with the aid of a core-conductor model that permits quantitative representation of cleft- and lateral surface membranes for individual preparations (Hellam & Studt, 1974).

3. The analysis led to the following conclusions. The slope conductance of unit membrane area (G_m) is quite low in the normal resting potential range; at -80 mV the value is 0·09 mmho/cm². G_m exhibits a marked decline with depolarization in the region positive to -88 mV, but only slight voltage-dependence at more negative potentials within the range of present observations. The capacitance of unit membrane area (C_m), from voltage ramp data collected near -90 mV, is 0·9 µF/cm².

4. Comparison with results of earlier studies indicates that the differences between present and previous experimental estimates of these membrane characteristics result from differences in modelling and structural analysis of tissue preparations.