Polymer chains extend and contract in response to changes in environmental conditions. The molecular behavior can be magnified by fabricating a three-dimensional network in a hydrogel. The polymeric hydrogels undergo abrupt volume changes in response to changes in environmental conditions, such as solvent composition, 1 electric field, 3 temperature, 4 and chemical concentration. 5 However, the observation of the extension and contraction of polymer chains in solution in a solvated state using microscopy is difficult due to the requirement for polymers to be dried. In the present study, the extension and contraction of the polymer chains in “polyelectrolyte brushes” 6-13 grafted on a nanoporous polymeric membrane were visualized using atomic force microscopy, which enables direct microscopic observation in water. Poly (methacrylic acid) which acted as the polyelectrolyte brush was grafted on track-etch porous polycarbonate membrane (DuPont Nuclepore membrane; average pore diameter 200 nm) by glow-discharge treatment. 14 Under low pH conditions, the poly (methacrylic acid) chain is protonated and contracted; under high pH conditions, the chain is deprotonated and extended. Pore size was estimated by measuring the rate of water permeation through the membrane (Figure 1a). The rate of water permeation through an ungrafted membrane was independent of pH. The rate of permeation through the grafted membrane was found to be less than that of the ungrafted membrane, but was dependent upon pH. The rate was high at low pH, but was nearly zero at neutral pH. These changes in permeation were repeatedly carried out by pH changes, because the polyelectrolyte brush was covelently grafted on the membrane.