Method and apparatus for controlling the dialysate flow in a dialysis device. A change of the value of a control factor, caused by a variation of a property of the dialysate or of the blood or by a change of the dialysate flow, is determined in order to control the dialysate flow. The control factor is a measure for the exchange of substances via the dialyzer and thus the effectiveness of the dialyzer. If the change of the value of the control factor exceeds a limit, the dialysate flow is increased. On the other hand, the dialysate flow is reduced if the change of the value of the control factor falls short of the limit range.