1. The voltage‐clamp technique was used to separate the effects of K depletion in the T‐system from the decline in K permeability during hyperpolarization, and to characterize the time‐ and voltage‐dependence of the latter. 2. K permeability due to the inward rectifier can be described as being proportional to a parameter which diminishes when the membrane is hyperpolarized beyond −120 mV. The parameter obeys first‐order kinetics. At 24° C, it can change with a time constant of 49 msec at −150 mV and 25 msec at −65 mV. At −200 mV the fall in membrane conductance due to the permeability change is to 30% of its initial value. The Q10 for the rate of conductance change at that potential is about 2·8. 3. It is estimated that K inward current can lower the average K concentration in the T‐system by more than 50%, and that, on the average, the space enclosed by the T‐system should be less than 0·8% of the fibre volume. Assuming the T‐system space to be 0·3% of the fibre volume, it is calculated that on the average, and during hyperpolarization to about −150 mV, no more than 20% of the initial current should flow across the surface membrane.
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