Alkali cation transport by tetrakis(ethoxycarbonylmethyl ether) 1 and tetrakis(methyl ether) 2 of p-tert-butylcalix[4]arene through soybean phospholipid bilayer membranes has been investigated by measurements of electric currents and dynamic 23Na NMR spectra. When the calix[4]arene ether 1 was incorporated into a planar phospholipid bilayer membrane which separated two chambers filled with 100 mM NaCl solutions, electric currents resulting from sodium ion fluxes across the planar bilayer membrane were generated upon applying external voltages between two chambers. On the other hand, incorporation of the methyl ether 2 into the planar bilayer did not generate electric currents resulting from sodium ion fluxes under the same experimental condition with 1. The current−voltage (I−V) relationships in XCl/XCl (X = Li, Na, K, Rb, Cs) systems showed that the calix[4]arene 1 selectively transported sodium ions through the phospholipid bilayer membrane. From the measurement of reversal potentials in XCl/NaCl system...