The invention relates to a blood treatment device comprising an extracorporeal circuit, a dialysate circuit, and a dialysis machine which has a blood-side chamber and a dialysate-side chamber which is separated from the blood-side chamber by a membrane. The invention is characterized in that the blood-side chamber is fluidically connected to the extracorporeal circuit, and the dialysate-side chamber is fluidically connected to the dialysate-circuit. The blood treatment device also comprises a preparation unit for the on-line production of a solution. The invention is characterized in that the blood treatment device has first means for carrying out a priming mode and second means for carrying out an initial treatment mode. The first means is designed to actuate the preparation unit such that a flushing solution with a pH value of ≥ 7.3 and preferably ≥ 7.4 is produced, and the extracorporeal circuit is filled with the flushing solution. The second means is designed to actuate the preparation unit such that dialysate with a pH value of ≥ 7.3 and preferably ≥ 7.4 is produced and the dialysate circuit is filled with the dialysate. The blood treatment device also has a control or regulating unit which operates the first means prior to the second means.