Abstract: An operating internal combustion engine with at least one cylinder (2), and a piston (12), which can move in an alternating manner therein, is used to compress a fuel mix in a combustion chamber (11). In order to determine the nitrogen oxide content in oxygen-containing exhaust gases, the quantity of fuel fed to the cylinder (2) and the air mass flowing in an induction pipe (15) are recorded and are fed to an electronic circuit (6). The center of gravity (S) of the combustion is determined from at least one current measured value for the engine operation, and the level of nitrogen oxide emissions is calculated from the value for the center of gravity (S) of the combustion, including the values for the recorded fuel quantity and air mass.