We have fabricated a molecular recognition ion gating membrane. This synthetic membrane spontaneously opens and closes its pores in response to specific solvated ions. In addition to this switching function, we found that this membrane could control its pore size in response to a known concentration of a specific ion. The membrane was prepared by plasma graft copolymerization, which filled the pores of porous polyethylene film with a copolymer of NIPAM (N-isopropylacrylamide) and BCAm (benzo[18]crown-6-acrylamide). NIPAM is well-known to have an LCST (lower critical solution temperature), at which its volume changes dramatically in water. The crown receptor of the BCAm traps a specific ion, and causes a shift in the LCST. Therefore, selectively responding to either K(+) or Ba(2+), the grafted copolymer swelled and shrank in the pores at a constant temperature between two LCSTs. The solution flux in the absence of Ba(2+) decreased by about 2 orders of magnitude over a solution flux containing Ba(2+). The pore size was estimated by the filtration of aqueous dextran solutions with various solute sizes. This revealed that the membrane changed its pore size between 5 and 27 nm in response to the Ba(2+) concentration changes. No such change was observed for Ca(2+) solutions. Furthermore, this pore size change occurred uniformly in all pores, as a clear cut-off value for a solute size that could pass through pores was always present. This membrane may be useful not only as a molecular recognition ion gate, but also as a device for spontaneously controlling the permeation flux and solute size.