A method for application 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 operated 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 varies the metered quantity during certain operating phases and compares the change in the measured value of at least one NOx sensor installed downstream of the SCR catalyst with an expected value.