A method for controlling an extracorporeal blood treatment apparatus which comprises: an interchangeable unit, which is divided by a semi-permeable membrane into a first chamber and a second chamber, wherein the first chamber is part of an extracorporeal blood circuit and the second chamber is part of a fluid system, a first pump for conveying a fluid from a first container at a first delivery rate into the second chamber of the interchangeable unit, a second pump for conveying a fluid from the second chamber of the interchangeable unit at a second delivery rate into a second container, a third pump for conveying a fluid at a third delivery rate from a third container or the first container into the extracorporeal blood circuit, first means for determining the sum of the weights of the first and third container or the weight of the first container, second means for determining the weight of the second container, wherein the pumps (10, 12, 16) for balancing the fluids are regulated as a function of the sum of the weights of the first and third container or the weight of the first container on the one hand and the weight of the second container on the other hand, characterised in that the weight reduction or weight increase of at least one of the containers 8, 13, 14) is measured in the time interval in which the pump (10, 12, 16) assigned to the respective container performs a preset number of revolutions or pumps strokes, the delivery quantity of the respective pump (10, 12, 16) at the preset number of revolutions or pump strokes is ascertained from the measured weight reduction or weight increase in the specific time interval, and the setpoint delivery quantity of the pump at the preset number of revolutions or pump strokes adopted in the drive of the respective pump (10, 12, 16) is compared with the measured delivery quantity of the pump at the preset number of revolutions or pump strokes, and the drive of the pumps (10, 12, 16) is based on the deviation of the adopted setpoint delivery quantity of the respective pump at the preset number of revolutions or pump strokes from the measured delivery quantity of the pump at the preset number of revolutions or pump strokes. an interchangeable unit, which is divided by a semi-permeable membrane into a first chamber and a second chamber, wherein the first chamber is part of an extracorporeal blood circuit and the second chamber is part of a fluid system, a first pump for conveying a fluid from a first container at a first delivery rate into the second chamber of the interchangeable unit, a second pump for conveying a fluid from the second chamber of the interchangeable unit at a second delivery rate into a second container, a third pump for conveying a fluid at a third delivery rate from a third container or the first container into the extracorporeal blood circuit, first means for determining the sum of the weights of the first and third container or the weight of the first container, second means for determining the weight of the second container, wherein the pumps (10, 12, 16) for balancing the fluids are regulated as a function of the sum of the weights of the first and third container or the weight of the first container on the one hand and the weight of the second container on the other hand, characterised in that the weight reduction or weight increase of at least one of the containers 8, 13, 14) is measured in the time interval in which the pump (10, 12, 16) assigned to the respective container performs a preset number of revolutions or pumps strokes, the delivery quantity of the respective pump (10, 12, 16) at the preset number of revolutions or pump strokes is ascertained from the measured weight reduction or weight increase in the specific time interval, and the setpoint delivery quantity of the pump at the preset number of revolutions or pump strokes adopted in the drive of the respective pump (10, 12, 16) is compared with the measured delivery quantity of the pump at the preset number of revolutions or pump strokes, and the drive of the pumps (10, 12, 16) is based on the deviation of the adopted setpoint delivery quantity of the respective pump at the preset number of revolutions or pump strokes from the measured delivery quantity of the pump at the preset number of revolutions or pump strokes.