A method of and apparatus for controlling the reducing agent in an exhaust gas aftertreatment system of an internal combustion engine with an exhaust gas duct in which an SCR catalyst is provided in the direction of flow of the exhaust gas. A reducing agent-generating system (RGS) comprising an NOx and CO/H2 production unit as well as a combined NOx storage/ammonia production unit in the standard gas path of the reducing agent-generating system. Ammonia is delivered upstream from the SCR catalyst to reduce nitrogen oxides, starting materials for producing the ammonia being fed at least in part to the NOx and CO/H2 production unit via a fuel inlet and an air inlet. An AGC matrix of the combined NOx storage/ammonia production unit is cooled at least intermittently by means of an inner and/or outer cooling device. Cooling results in the longest possible section of the running length (L) and thus the largest possible volume percent of the AGX matrix within the combined NOx storage/ammonia production unit having an optimum temperature range regarding the highest possible ammonia yield, which overall contributes to a great ammonia production rate.