1. A three-electrode voltage clamp method was used to investigate the mechanism of the fall in resting potassium permeability which occurs under extreme hyperpolarization in frog sartorius muscle fibres. 2. Experiments were performed which show that this permeability change is due to a potential dependent block by Na+ ions present in the external solution. 3. Inward K-currents recorded on hyperpolarization turned off exponentially with time. In the presence of Na the steady-state current-voltage relation had a region of negative slope beyond -140 mV. This negative-slope region was removed when Na was replaced by TMA, Tris or Li. Increasing [Na] to 140 mM shifted the negative-slope region to less negative membrane potentials; reducing [Na] to 14 mM shifted the region to more negative potentials. 4. The time constant for the turn-off of the currents (tau) was the same in Na and TMA-containing solutions at membrane potentials positive to -140 mV. At more negative membrane potentials the tau s in Na became progressively shorter than those in TMA. Increasing [Na] to 140 mM (from 70 mM) gave smaller tau s at all potentials. 5. If fibres were hyperpolarized to -240 mV and then repolarized to -160 mV in 70 mM-Na the current recorded during the second pulse turned on with time, often reaching a value greater than that at the end of the first pulse. This behaviour was removed when Na was replaced by TMA or Tris. 6. An estimate of the steady-state relationship between the degree of block and membrane potential was obtained, and could be fitted by an expression for a potential-dependent ionic block with a very low affinity binding site for Na+ in the membrane. 7. The recovery after hyperpolarization of K-currents at the holding potential was examined in two-pulse experiments. In 70 mM-TMA recovery occurred at the same rate whether the initial hyperpolarization was to -120 or to -210 mV. In 70 mM-Na recovery after an initial pulse to -120 mV occurred at the same rate as in TMA, but recovery after a pulse to -210 mV occurred about 9 times faster. These results are consistent with depletion of K from the lumen of the T-system dominating the turn-off of K currents in TMA and in Na for the hyperpolarization to -120 mV, but a different mechanism being involved for the -120 mV pulse in Na. 8. A three-compartment model is presented which attempts to describe the depletion of K from the T-system. The model accurately predicts the time-course for the decline of inward K-currents, both in 10 and 80 mM-K solutions.