This paper discusses the alkaline ion (Na+) role in the uphill transport of amino acids through a bulk liquid membrane. The aqueous phases (source phase -S and receiving phase -R) are made up of equimolar concentrations of amino acid (4.38 mM p-aminobenzoic acid (PABA)) and alkaline ion (75 mM Na+). A chloroform solution containing 5 mM dibenzo-18-crown-6 (DB18C6) represents the bulk liquid membrane (M). The data obtained show that at the S-M interphase, the amino acid is coupled with the carrier via the H3N+ group rather than being transported to the R-M interphase, where Na+ substitutes the amino acid. If Na+ is absent, the amino acid is transported to the opposite direction. These results support the hypothesis that the presence of Na+ ion in the aqueous phases assures the ’biological’ direction of aminobenzoic acids transport through membranes.