The behavior of sodium ions passing through a CM1 cation exchange membrane was studied with electroconductive and diffusional membrane permeabilities based on nonequilibrium thermodynamics to remove sodium ion from bile acid solution. These membrane permeabilities of sodium ion in order of amplitude showed sodium chloride > sodiumtaurocholate > sodium cholate systems in all cases. In the sodium cholate and sodium taurocholate systems, the curves relating the electroconductive membrane permeability with concentration showed distinct changes in gradient at around the concentration of primary micelle formation of each bile acid solution. This was attributable to an increase in membrane/solution distribution of bile acid anion on basis of the micelle formation. Using diffusional dialysis, 7.3% of sodium ion was removed from rat serum. Sodium ion was significantly reduced with increasing total bile acid concentration in rat serum (P < 0.05; correlation coefficient = -0.606). (C) 2003 Elsevier B.V. All rights reserved.