[en] The modification of the ultraviolet blocking of sodium channels and of the ultraviolet-induced potential shift of the gating parameters by means of the sulfhydryl compounds l-cysteine and 2-mercaptoethanol was investigated in the node of Ranvier of Rana esculenta under voltage-clamp conditions. The UV wavelength was 280 nm. The radiation-induced potential shift of the voltage-dependent gating parameters was prevented or even reversed by the action of the sulfhydryl compounds (internal application), while the blocking effect was not affected. It is concluded that the two radiation effects are caused by two separate photoreactions. Internally applied N-ethylmaleimide, binding specifically to protein-SH groups, exhibits an effect similar to the ultraviolet-induced potential shift, without affecting the maximum sodium permeability. Therefore, the ultraviolet-induced potential shift might be caused by a photocatalyzed oxidation of -SH groups of membrane proteins changing the surface charge density at the inner side of the nodal membrane