Abstract: A system for reducing NO.sub.x gases found in the exhaust stream of the internal combustion engine is provided which includes an internal combustion engine having associated therewith a means for supplying fuel and oxygen to the engine, a manifold system for directing or channeling the exhaust gases from the engine, and a NO.sub.x reducing catalyst assembly communicating with the manifold which includes a housing having a NO.sub.x reducing catalytic structure of apertured thin metal foil positioned therein which includes, in one aspect, a plurality of sheets of expanded metal foil wherein at least one sheet of expanded metal foil has at least some openings therein which are of a different size than at least some openings in a juxtapositioned sheet of expanded metal foil. In another aspect, it includes a sheet of expanded metal foil having at least some openings of different sizes.