A method of checking and/or monitoring the correct operation of an adding device of a medical apparatus includes detecting a characteristic signal course being generated by the adding device. The medical apparatus includes an extracorporeal circuit with which the adding device is connected such that an agent can be introduced into the extracorporeal circuit via the adding device. The pressure loss in the extracorporeal circuit is measured and evaluated for checking and/or monitoring the correct operation of the adding device. The adding device for this purpose generates an oscillating pressure course.