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1. The effects of low temperature on the positive dynamic current (mainly Cl^- current) of sheep cardiac Purkinje fibres were studied using the `voltage clamp' technique.

2. Cooling of Purkinje fibres from 37° C to room temperature (25° C) slowed the time course of the current and decreased its peak amplitude. The Q₁₀ of the decay of the current was 2·67 over 25-37° C. Removal of inactivation was also delayed and the Q₁₀ of the rate constant was 2·62 over 30-37° C.

3. The decreased peak amplitude of the current on cooling could not be restored by pre-step hyperpolarization. Thus the curve for the steady-state inactivation was simply depressed downward. The Q₁₀ of the maximum current response was 3·05 over 25-37° C.

4. During deep hypothermia (less than 10° C), no Cl^- current was demonstrated by depolarization up to + 60 mV.

5. The high sensitivity to temperature suggests that the current contributes significantly to the prolongation of action potentials during low temperature, but its quantitative role cannot be evaluated without simultaneous evaluation of other plateau currents.