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1. When one hyperpolarizes a muscle fibre by passing electric current, the K conductance declines with time. Voltage-clamp experiments on frog sartorius muscle fibres showed that two components contribute to this decline.

2. A rapid component operates when the fibre is hyperpolarized to potentials more negative than -120 mV. Decline by this mechanism is reversed completely within 200 msec. The large effect of temperature on the kinetics of this process indicates that it represents a time-dependent membrane permeability change.

3. A slow component operates also at less negative potentials. Recovery at -65 mV takes place with half-times of about 0·4 sec. The Q₁₀ for the rate of recovery is 1·3, indicating that this process is diffusion limited.

4. After prolonged hyperpolarization to potentials positive to -120 mV, membrane current at the resting potential is outward and persists for several seconds. At that time, the potential measured in the absence of membrane current is shifted in the negative direction by 3-5 mV.

5. This shift and the time course of currents near the resting potential after hyperpolarization as well as the Q₁₀ of 1·3 indicate that the slow process is due to changes in tubular K concentration and not to a time-dependent membrane permeability change.

6. At potentials less negative than -120 mV, tubular depletion can satisfactorily account for the decline of K conductance. At more negative potentials, the decline appears to be due to both depletion and a permeability change.

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