The flow of air to feed oxygen to the cathode of each plate in a proton exchange membrane fuel cell (PEMFC) is studied for a 300W stack in a realistic 3D configuration. Two configurations for gas income are solved, a "U" shape, where both the inlet and outlet of the air collectors are at the same end plate, and a "Z" shape, where inlet and outlet are at opposite sides of the stack. Under a simplified assumption for the flow of oxygen entering the gas diffusion layer of each cell, detailed mass flow and pressure distributions are shown, including the possibility of a turbulent flow inside the main collectors. (C) 2008 Elsevier B.V. All rights reserved.