The effect of the nonionic detergent Triton X-100 (TX-100) on the Na permeability (PNa) properties of the nodal membrane in myelinated nerve fibres of Xenopus laevis was analysed with potential clamp technique. Application of TX-100 caused a rapid initial decrease in PNa that was reversible at wash out as well as a slow irreversible block. Both effects dependend on [TX-100] and duration of exposure. The reversible reduction of PNa at the steady state was 50% at 40--60 micrometer TX-100. The slope of the Hill plot for the reaction was 1.75 indicating a deviation from a first order reaction. The equilibrium dissociation constant (KD) for n = 1.75 was 0.9 X 10(-7) (M1.75). KD calculated from the rate constants for onset and offset of the reversible reaction (KD = k2/k1) was 1.5 X 10(-7) (M1.75). The possibility that the action of TX-100 involves membrane proteins is discussed.