Binding of potassium ions through an access channel from the cytoplasmic side of Na + ,K +-ATPase and the effect of pH and magnesium ions on this process have been studied. The studies were carried out by a previously developed method of measuring small increments of the admittance (capacitance and conductivity) of a compound membrane consisting of a bilayer lipid membrane with adsorbed membranes fragments containing Na + ,K +-ATPase. The capacitance change of the membrane with the Na + ,K +-ATPase was induced abruptly by release of protons from a bound form (caged H +) upon a UV-light flash in the absence of magnesium ions. The change of admittance consisted of an initial fast jump and a slow relaxation to a stationary value within a time of about 1-2 s. The kinetics of the capacitance relaxation depended on pH and the concentration of magnesium and potassium ions. The dependence of the rapid capacitance jump on the potassium concentration corresponded to the predictions of the model developed earlier that describes binding of sodium or potassium ions in competition with protons. The effect of magnesium ions can be explained by the assuming that they bind to the Na + ,K +-ATPase and affect binding of potassium ions because of either changes in protein conformation or the creation of an electrostatic field in the access channel on the cytoplasmic side.