A membrane-electrode assembly and polymer electrolyte fuel cells and methods of production thereof, in which a polymer membrane, containing at least one basic polymer membrane, is sandwiched between two flat gas diffusion electrodes each of which is loaded with a dopant, whereby after reaching a mass transport equilibrium for the exchange of the dopant between the gas diffusion electrodes and the polymer membrane, the polymer membrane has a conductivity of at least 0.1 S/m at a temperature of no less than 25.degree. C.