A single relaxation of order of 0.1 s was found in the car boxymethyl (CM) Sephadex suspension containing a basic amino acid, lysine or arginine, by the pressure-jump method with electric conductivity detection. However, no relaxation was observed in the suspension containing histidine. Although the pH of the suspension increased with increasing concentration of the added amino acid, the relaxation was observed in the pH range of 4.5 to 7.0. In this pH range, most of lysine and arginine exist as apparently univalent cations, while histidine exists as apparently neutral zwitterions as well as apparently univalent cations, The relaxation times became fast with increasing concentrations of the univalent cations of lysine and arginine, On the other hand, these cations were strongly adsorbed on the CM Sephadex particle which had a soft spongy structure consisting of flexible dextran chains, On the basis of the dependences of the relaxation times and the adsorbed amounts of the amino acids on the concentrations of the added amino acids, the relaxation was attributed to a conformational change of the Sephadex particle accompanying the adsorption-desorption of the apparently univalent amino acid cations.