The ionic currents of the frog (Rana ridibunda) nodal membrane were measured under voltage clamp conditions. The membrane being +40 mV. The replacing of the external Na+-ions to K+- and NH4+-ions have showed that the relative permeabilities of the veratrine-modified channels calculated from the constant field theory are arranged in the following row: PNa:PK:PNH4 = 1:0.29:0.61, which differs from the same row for the normal channels. The decreasing of the slope of current-voltage relations of the modified channels with the replacing of Na+-ions to K+- and NH4+-ions is the evidence of a more strong binding of these ions to the external mouth of the modified channel compared to the binding of Na+-ions.