pH-responsive polypeptide, poly(Leu.Gln(EtNHz)), membranes consisting of hydrophobic leucine (Leu) and hydrophilic N(omega)-(beta-(aminoethyl)-L-glutamine) (Gln(EtNH2)) residues have been prepared and pH-induced conformational changes of the polypeptide in membranes and their relation to membrane swelling and solute permeation properties were investigated. IR spectra of the membrane containing 31% Gln(EtNH2) residues indicated an alpha-helix to random coil transition by decreasing pH with the midpoint of the transition at ca. pH 4.7. Moreover, fluorescence measurements of N-phenyl-1-naphthylamine entrapped in the membrane showed that a strong association between the hydrophobic leucine residues could be formed when the conformation of the polypeptide was converted from a rigid alpha-helix to a flexible charged coil below the midpoint pH of the transition. On the other hand, the degree of hydration of the poly(Leu.Gln(EtNH2)) membranes with various Gln(EtNH2) content was steeply increased with decreasing pH from the alkaline to weak acid pH values, resulting from the conformational transition following an ionization of Gln(EtNH2) residues. However, the further decrease in pH induced a remarkable deswelling of the membranes, which could be explained in terms of the formation of hydrophobic clusters between leucine moieties in the induced charged coil conformation yielding intermolecular bridges in the membranes. The styrene glycol permeabilities and/or diffusibilities through the membrane with 31 % Gln(EtNH2) residues, as a result, could be effectively regulated by the pH-induced swelling and deswelling of the membrane; i.e., the permeability of the solute could be significantly enhanced only at around pH 4.5. It is also found that, similar to the case of the membrane system, the solute release rate from a capsule membrane of poly(Leu.Gln(EtNH2)) containing 22% Gln(EtNH2) residues could be reversibly controlled by the external pH.