Abstract: A process and device are disclosed for measuring blood platelet aggregation or blood coagulation. The blood flows through an opening in a part and the occlusion of the opening is measured. The pressure fall (.DELTA.P) formed during occlusion of the opening is measured at predetermined time intervals (dt) and the volume flow (I) is changed to correspond to a predetermined function which simulates the flow resistance (R.sub.v) of a capillary upstream of the opening. Alternatively, the pressure (.DELTA.P) is kept constant during the predetermined time intervals (dt) and afterwards, when the volume flow (I) has diminished by a certain value, the pressure is adjusted until it corresponds to the function.