A haemodialysis device having an emergency activating function is provided. In order to avoid critical drops in blood pressure during a dialysis treatment, the emergency activating function makes it possible for the operating staff to initiate with a single control function a blood pressure measurement and a delivery of substance into the patient's blood in the event of a hypotensive episode and at the same time to have the introduction of these measures by the haemodialyser confirmed clearly through a display having an outward appearance different from that displayed during normal operations. In addition, further automatically introduced process steps can be provided. The emergency activating function speeds up the implementation of the measures to be introduced and, at the same time prevents incorrect operation and makes it possible for the operating staff to attend directly to the patient.