Voltage clamped single nerve fibres of the frogPanta esculentawere treated with the amino groups specific reagents ethyl acetimidate and isethionyl acetimidate. Ethyl acetimidate is lipid soluble, relatively non-polar and can penetrate a membrane. Isethionyl acetimidate is lipid-insoluble, polar and membrane-impermeant. Treatment with ethyl acetimidate caused an irreversible reduction of Na currents and a shift of the voltage dependence of the steady-state sodium inactivation,h∞(E), in the hyperpolarizing direction. The voltage dependence of sodium activation was much less affected and a small shift into the depolarizing direction was observed. By contrast, the non-permeant reagent did not reduce the sodium currents and the shifts of theh∞(E) curve were smaller than the shifts caused by ethyl acetimidate. Furthermore, a small shift of the voltagedependence of activation in the hyperpolarizing direction was observed. As the modification procedure with imidoesters does not cause a change of net charge, the results cannot be explained by an effect of surface charge modification, rather, they seem to be due to a direct effect of amino group modification on the voltage dependence of inactivat