A method for diagnosing a metering valve for metering a reagent into the exhaust gas region of an internal combustion engine. The metering valve is actuated by a pulse-width-modulated metering valve actuation signal with a certain duty factor for setting the metering rate. A reagent pump places the reagent at a reagent pressure, and the reagent pump is operated with a pulse-width-modulated pump actuation signal with a certain duty factor. Diagnosis of the metering valve is carried out on the basis of an evaluation of the increase in the metering valve actuation signal pulse duty factor after a predefined increase in the metering rate. The reagent filling level of an SCR catalytic converter arranged in the exhaust gas region is taken into account. At the start of the diagnosis, the reagent storage capacity of the SCR catalytic converter is checked.