The resistance of the Nafion(R) 117 membrane in H-2/O-2 and H-2/air polymer electrolyte fuel cells (PEFCs) has been measured in situ using fast current pulses. The dependence of the membrane resistance on current density, temperature, pressure and flow-field design was investigated. It was found that, independent of other variations, the resistance increases with increasing current density. When the current density in the cell is increased from 0.2 to 0.7 A cm(-2), the membrane resistance increases by up to 22%. Even on open circuit the resistance at 60 degrees C is 15%-35% higher than that measured ex situ, indicating that the membrane is not fully hydrated under the fuel cell operating conditions. The resistance on open circuit also depends on the design of the flow field. In a design with forced gas convection the resistance at 60 degrees C is substantially higher (210 m Omega cm(2)) than in a design without forced convection (186 m Omega cm(2)).