A fuel cell assembly having manifold means for providing fuel and air to, and removing spent fuel and air from, flow passageways across the anodes and cathodes in a fuel cell stack. The sizes and proportions of the supply and return manifolds are optimized, and the total cross-sectional area of the return manifold is about twice the cross-sectional area of the supply manifold. The pressure drop in the manifolds is less than about one-quarter of the total pressure drop across the anode and cathode passageways in the stack, which ratio may be attained by adjusting the thickness of the anode and cathode spacers and/or the size of the chimneys. Widthwise uniformity of flow across the anodes and cathodes is improved by forming each of the manifolds as a plurality of smaller, parallel flow conduits.