A method for use in conjunction with an exhaust gas post-treatment system for metering an ammonia-releasing reducing agent into the exhaust gas stream of an internal combustion engine installed in a motor vehicle and operating with excess air. A control unit meters the quantity of reducing agent as a function of a stored model and, during the operation of the internal combustion engine, determines, by comparison of a value proportional to the nominal emission or the nominal conversion stored in the control unit of the internal combustion engine for the associated operating point of the internal combustion engine and/or of the exhaust gas post-treatment system with a value proportional to the actual emission or the actual conversion metrologically determined by the control unit.