A device for the extracorporal blood treatment in a single-needle operating mode includes means for delivering blood into, and means for delivering blood out of means for collecting blood, and a controller for setting the respective delivery rates of the means for the delivery of blood. The operating mode of the means for delivering blood is continually switched between an arterial and a venous phase by the controller, wherein during the arterial phase the delivery rate Qb of the means for delivering blood is greater than the delivery rate Qsn of the means for delivering blood such that blood may be withdrawn from the patient during the arterial phase, and during the venous phase the delivery rate Qb is smaller than the delivery rate Qsn such that blood may be administered to the patient during the venous phase.