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 degrees 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 Q(10) 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.