Abstract: The ultra-filtration in a hemodialysis operation is controlled by a control apparatus which is connected to a balancing device (1) and to a dosing device (9) to form a closed loop control circuit for the metered supply of dialysis concentrate into the balancing device and for the metered supply of fresh dialysis solution into the dialyzer. The closed loop control circuit controls the alternating supply of fresh dialysis solution into one chamber of a balancing device and the withdrawal of used-up dialysis solution from the other chamber of the balancing device. The closed loop control circuit also controls a pump for the metered withdrawal of used-up dialysis solution out of the dialysis circulatory system. The dosing device operates as a proportionality device to produce fresh dialysis solution from water and dialysis concentrate whereby the concentrate is dosed and the dialysis solution is also dosed or metered.