A method and a device are provided for operating an internal combustion engine which allow for an improved diagnosis of the valve mechanism of cylinders of the internal combustion engine. For this purpose, a variable characteristic of a suction performance of a cylinder of the internal combustion engine is ascertained. The variable characteristic of the suction performance is ascertained as a function of the mass flow flowing into an intake manifold of the internal combustion engine and of a change of the intake manifold pressure during an intake phase of the cylinder.