1. Inward rectification of frog muscle membrane was analysed with the Vaseline gap method. 2. Hyperpolarization, under voltage clamp, produced inward potassium currents, which had a component that activated with a time constant, tau K. 3. The activation time constant tau K of the inward potassium currents was voltage dependent. For a given external potassium concentration, the time constant was maximal for potentials near the potassium equilibrium potential, EK. 4. The potassium chord conductance gK, had a sigmoidal voltage dependency, increasing initially e-fold per 11.6 mV of hyperpolarization. 5. When the internal potassium concentration was fixed, raising external potassium induced a shift of the tau K-V and the gK-V relations in the positive direction along the voltage axis. That shift was comparable to the change in EK. 6. No shift of the tau K-V and the gK-V relations was observed when the internal potassium was reduced from 150 to 50 mM. 7. Changes of internal sodium concentration between 5 and 100 mM did not significantly effect the magnitude of inward rectification.